Optical scanning device that includes waveguides

ABSTRACT

An optical scanning device includes a waveguide array including a plurality of waveguides arranged in a first direction. Each waveguide includes: an optical waveguide layer that propagates light supplied to the waveguide in a second direction intersecting the first direction; a first mirror having a first reflecting surface intersecting a third direction; and a second mirror having a second reflecting surface that faces the first reflecting surface. The optical waveguide layer is located between the first and second mirrors and has a variable thickness and/or a variable refractive index for the light. The width of the first mirror and the width of the second mirror are each larger than the width of the optical waveguide layer. The first mirror has a higher light transmittance than the second mirror and allows part of the light propagating through the optical waveguide layer to be emitted in the third direction.

RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.16/116,499, filed on Aug. 29, 2018, which is a Continuation ofPCT/JP2017/029704, filed on Aug. 21, 2017, which is a Continuation ofPCT/JP2017/000709, filed Jan. 12, 2017, which in turn claims the benefitof Japanese Application No. 2017-090049, filed on Apr. 28, 2017,Japanese Application No. 2017-050501, filed Mar. 15, 2017, JapanesePatent Application No. 2017-005833, dated Jan. 17, 2017, and JapaneseApplication No. 2016-191317, filed on Sep. 29, 2016, the entiredisclosures of which applications are incorporated by reference herein.

BACKGROUND 1. Technical Field

The present disclosure relates to an optical scanning device thatincludes waveguides.

2. Description of the Related Art

Various devices capable of scanning a space with light have beenproposed.

International Publication No. WO2013/168266 discloses a structure thatcan perform optical scanning using a driving unit for rotating a mirror.

Japanese Unexamined Patent Application Publication (Translation of PCTApplication) No. 2016-508235 discloses an optical phased array includinga plurality of nanophotonic antenna elements arranged in two dimensions.Each antenna element is optically coupled to a corresponding variableoptical delay line (i.e., a phase shifter). In this optical phasedarray, a coherent light beam is guided to each antenna element through acorresponding waveguide, and the phase of the light beam is shifted by acorresponding phase shifter. In this manner, an amplitude distributionof a far-field radiation pattern can be changed.

Japanese Unexamined Patent Application Publication No. 2013-16591discloses a light deflection element including: a waveguide including anoptical waveguide layer through which light is guided and firstdistributed Bragg reflectors formed on the upper and lower surfaces ofthe optical waveguide layer; a light inlet for allowing light to enterthe waveguide; and a light outlet formed on a surface of the waveguideto allow the light entering from the light inlet and guided through thewaveguide to be emitted.

SUMMARY

One non-limiting and exemplary embodiment provides a novel opticalscanning device having a relatively simple structure capable of opticalscanning.

In one general aspect, the techniques disclosed here feature an opticalscanning device including a waveguide array including a plurality ofwaveguides arranged in a first direction. Each of the plurality ofwaveguides includes: an optical waveguide layer that propagates lightsupplied to the waveguides in a second direction intersecting the firstdirection; a first mirror extending in the second direction and having afirst reflecting surface that intersects a third direction; and a secondmirror extending in the second direction and having a second reflectingsurface facing the first reflecting surface of the first mirror. Thethird direction intersects a virtual plane parallel to the first andsecond directions. The optical waveguide layer is located between thefirst mirror and the second mirror and has a variable thickness and/or avariable refractive index for the light. The width of the first mirrorin the first direction and the width of the second mirror in the firstdirection are each larger than the width of the optical waveguide layerin the first direction. The first mirror has a higher lighttransmittance than the second mirror and allows part of the lightpropagating through the optical waveguide layer to pass through and beemitted in the third direction. The refractive index of the opticalwaveguide layer and/or the thickness of the optical waveguide layer ischanged to change the third direction that is an emission direction ofthe part of the light.

According to an aspect of the present disclosure, one-dimensionaloptical scanning or two-dimensional optical scanning can be achievedusing a relatively simple structure.

It should be noted that general or specific embodiments of the presentdisclosure may be implemented as a device, a system, a method, anintegrated circuit, a computer program, a storage medium, or anyselective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing the structure of anoptical scanning device in an exemplary embodiment of the presentdisclosure;

FIG. 2 is an illustration schematically showing an example of across-sectional structure of one waveguide element and light propagatingtherethrough;

FIG. 3 is an illustration schematically showing a computational modelused for a simulation;

FIG. 4A shows the results of computations of the relation between therefractive index n_(w) of an optical waveguide layer and the emissionangle θ of light with a mode order of m=1 when the thickness d of theoptical waveguide layer is 704 nm;

FIG. 4B shows the results of computations of the relation between therefractive index n_(w) of the optical waveguide layer and the emissionangle θ of light with a mode order of m=1 when the thickness d of theoptical waveguide layer is 446 nm;

FIG. 5 is an illustration schematically showing an example of an opticalscanning device that can implement one-dimensional scanning using asingle waveguide element;

FIG. 6A is a cross-sectional view schematically showing an example(comparative example) of a structure in which light is inputted to thewaveguide element;

FIG. 6B is a cross-sectional view schematically showing an example(comparative example) of a structure in which light is inputted to thewaveguide element through an optical fiber inclined at an angle θ_(in)with respect to the direction normal to a mirror;

FIG. 7 is a graph showing changes in coupling efficiency when theincident angle θ_(in) of light was fixed and the refractive index n_(w)of a waveguide was changed to change the emergent angle flout of thelight;

FIG. 8 is an illustration schematically showing connections between aplurality of first waveguides formed on a substrate and a plurality ofsecond waveguides formed on another substrate;

FIG. 9 is a cross-sectional view of a waveguide element in the YZ plane,schematically showing a structural example in which spacers are disposedon both sides of an optical waveguide layer located between a firstmirror and a second mirror;

FIG. 10 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing a structural example of a waveguidearray prepared by arranging a plurality of the waveguide elements inFIG. 9 in the Y direction;

FIG. 11 is an illustration schematically showing propagation of guidedlight in the X direction within an optical waveguide layer;

FIG. 12 is a cross-sectional view schematically showing part of thestructure of an optical scanning device in an exemplary embodiment ofthe present disclosure;

FIG. 13 is a cross-sectional view schematically showing another exampleof the structure of the optical scanning device;

FIG. 14 is a cross-sectional view schematically showing yet anotherexample of the structure of the optical scanning device;

FIG. 15 shows an example in which light enters an optical waveguidelayer sandwiched between two multilayer reflective films, as in theexample shown in FIG. 12;

FIG. 16A shows an example in which light is introduced into a firstwaveguide through a grating disposed on a surface of the firstwaveguide, as in the example shown in FIG. 13;

FIG. 16B shows an example in which light is inputted from an end surfaceof the first waveguide;

FIG. 16C shows an example in which light is inputted from a laser lightsource disposed on the surface of the first waveguide through thesurface;

FIG. 17 shows the d₂ dependence of the coupling efficiency of guidedlight from a waveguide to a waveguide when n_(w1) is 1.45, d₁ is 1.27μm, and the wavelength λ is 1.55 μm;

FIG. 18 shows the results of computations performed using the samemethod as in FIG. 17 except that n_(w1) was changed to 3.48 and d₁ waschanged to 0.5 μm;

FIG. 19 is a graph showing the above results classified by whether thecoupling efficiency is 0.5 or more or less than 0.5, with the horizontalaxis representing d₂/d_(cutoff) and the vertical axis representing therefractive index ratio (|n_(w1)−n_(w2)|/n_(w1));

FIG. 20 is an illustration showing a structure in which the center, withrespect to the direction of thickness, of an optical waveguide layer ofa first waveguide is offset by Δz from the center, with respect to thedirection of thickness, of an optical waveguide layer of a secondwaveguide;

FIG. 21 is a graph showing the Δz dependence of the coupling efficiencyof light from a first waveguide to a second waveguide;

FIG. 22A shows the d₂ dependence of the coupling efficiency when n_(w1)is 2.2, d₁ is 0.7 μm, and the wavelength λ is 1.55 μm;

FIG. 22B shows the d₂ dependence of the coupling efficiency when n_(w1)is 3.48, d₁ is 0.46 μm, and the wavelength λ is 1.55 μm;

FIG. 23A is an illustration showing a computational model used tocompute propagation of light with a different mode order;

FIG. 23B is an illustration showing the results of computations ofpropagation of the light with the different mode order;

FIG. 24A is a cross-sectional view showing an optical scanning device inanother embodiment;

FIG. 24B is a graph showing the results of computations of the gap widthdependence of the coupling efficiency;

FIG. 25A is an illustration showing a cross section of a waveguide arraythat emits light in a direction perpendicular to the emission surface ofthe waveguide array;

FIG. 25B is an illustration showing a cross section of a waveguide arraythat emits light in a direction different from the directionperpendicular to the emission surface of the waveguide array;

FIG. 26 is a perspective view schematically showing a waveguide array ina three-dimensional space;

FIG. 27A is a schematic diagram showing how diffracted light is emittedfrom the waveguide array when p is larger than λ;

FIG. 27B is a schematic diagram showing how diffracted light is emittedfrom the waveguide array when p is smaller than λ;

FIG. 27C is a schematic diagram showing how diffracted light is emittedfrom the waveguide array when p=λ/2;

FIG. 28 is a schematic diagram showing an example of a structure inwhich a phase shifter is connected directly to a waveguide element;

FIG. 29 is a schematic diagram showing a waveguide array and a phaseshifter array as viewed in a direction normal to a light-emissionsurface (the Z direction);

FIG. 30 is an illustration schematically showing an example of astructure in which waveguides of phase shifters are connected to opticalwaveguide layers of waveguide elements through additional waveguides;

FIG. 31 is an illustration showing a structural example in which aplurality of phase shifters arranged in a cascaded manner are insertedinto an optical divider;

FIG. 32A is a perspective view schematically showing an example of thestructure of a first adjusting element;

FIG. 32B is a perspective view schematically showing another example ofthe structure of the first adjusting element;

FIG. 32C is a perspective view schematically showing yet another exampleof the structure of the first adjusting element;

FIG. 33 is an illustration showing an example of a structure in which awaveguide element is combined with an adjusting element including aheater formed of a material having high electrical resistance;

FIG. 34 is an illustration showing a structural example in which amirror is held by support members formed of an easily deformablematerial;

FIG. 35 is an illustration showing an example of a structure in which amirror and/or a mirror is moved by an electrostatic force generatedbetween electrodes;

FIG. 36 is an illustration showing a structural example in whichelectrodes that generate an attractive force are disposed in portionsthat do not impede propagation of light;

FIG. 37 is an illustration showing an example of a piezoelectric elementcontaining a piezoelectric material;

FIG. 38A is an illustration showing a structural example of a supportmember having a unimorph structure using the piezoelectric element shownin FIG. 37;

FIG. 38B is an illustration showing an example of a state in which thesupport member is deformed by applying a voltage to the piezoelectricelement;

FIG. 39A is an illustration showing a structural example of a supportmember having a bimorph structure using the piezoelectric elements shownin FIG. 37;

FIG. 39B is an illustration showing an example of a state in which thesupport member is deformed by applying a voltage to the piezoelectricelements on both sides;

FIG. 40 is an illustration showing an example of an actuator in whichthe support members shown in FIG. 38A are disposed on both sides of amirror;

FIG. 41A is an illustration showing the inclination of a forward endthat occurs in a unimorph-type actuator;

FIG. 41B is an illustration showing an example in which twounimorph-type support members having different expansion-contractiondirections are connected in series;

FIG. 42 is an illustration showing an example of a structure in which aplurality of first mirrors held by a support member (i.e., an auxiliarysubstrate) are collectively driven by an actuator;

FIG. 43 is an illustration showing a structural example in which oneplate-shaped first mirror is used for a plurality of waveguide elements;

FIG. 44 is an illustration showing an example of a structure in whichcommon wiring lines are led from electrodes of waveguide elements;

FIG. 45 is an illustration showing an example of a structure in whichthe wiring lines and some of the electrodes are shared;

FIG. 46 is an illustration showing an example of a structure in whichcommon electrodes are provided for a plurality of waveguide elements;

FIG. 47 is an illustration schematically showing an example of astructure in which waveguides are integrated into a small array while alarge arrangement area is allocated for a phase shifter array;

FIG. 48 is an illustration showing a structural example in which twophase shifter arrays are disposed on respective sides of a waveguidearray;

FIG. 49A shows a structural example of a waveguide array in which anarrangement direction d1 of waveguide elements is not orthogonal to anextending direction d2 of the waveguide elements;

FIG. 49B shows a structural example of a waveguide array in whichwaveguide elements are arranged at non-regular intervals;

FIG. 50A is an illustration schematically showing an optical scanningdevice in an embodiment;

FIG. 50B is a cross-sectional view of the optical scanning device in theXZ plane shown by one of broken lines in FIG. 50A;

FIG. 50C is a cross-sectional view of the optical scanning device in theYZ plane shown by the other one of the broken lines in FIG. 50A;

FIG. 51A is an illustration showing a structural example in which adielectric layer is disposed between a second mirror and a waveguide;

FIG. 51B is an illustration showing a structural example in which asecond dielectric layer is disposed on the first waveguide;

FIG. 52 is an illustration showing a structural example in which thesecond mirror is not disposed in a region between the first waveguideand the substrate;

FIG. 53 is an illustration showing a structural example in which,between the first waveguide and the substrate, the second mirror isthinner than the second mirror in the structural example in FIG. 51B;

FIG. 54A is an illustration showing a structural example in which thethickness of the second mirror varies gradually;

FIG. 54B is an illustration showing a structural example in which anupper electrode, a first mirror, and a second substrate are disposed soas to extend over a protective layer of the first waveguide and theoptical waveguide layer of the second waveguide;

FIG. 54C is an illustration showing part of a production process in thestructural example in FIG. 54B;

FIG. 55 is an illustration showing a YZ-plane cross section of aplurality of second waveguides in an optical scanning device having thestructure shown in FIG. 54B;

FIG. 56 is an illustration showing a structural example in which thefirst waveguide and the second waveguide are reflective waveguides;

FIG. 57 is an illustration showing a structural example in which theupper electrode is disposed on the upper surface of the first mirror andthe lower electrode is disposed on the lower surface of the secondmirror;

FIG. 58 is an illustration showing an example in which the firstwaveguide is separated into two portions;

FIG. 59 is an illustration showing a structural example in whichelectrodes are disposed between adjacent optical waveguide layers;

FIG. 60 is an illustration showing a structural example in which thefirst mirror is thick and the second mirror is thin;

FIG. 61 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing a structural example of a waveguidearray in an embodiment in which a plurality of waveguide elements arearranged in the Y direction;

FIG. 62 is a graph showing the relation between the ratio of light loss(L^((β)) _(loss)) and y₁ when β=1,000;

FIG. 63 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing another example of the waveguide arrayin the present embodiment in which the plurality of waveguide elementsare arranged in the Y direction;

FIG. 64A is a graph showing the results of computations of an electricfield intensity distribution in the structural example in FIG. 10;

FIG. 64B is a graph showing the results of computations of an electricfield intensity distribution in the structural example in FIG. 63;

FIG. 65 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing a structural example in an embodiment inwhich spacers having different refractive indexes are present;

FIG. 66 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing a structural example of a waveguideelement in a modification of the present embodiment;

FIG. 67 is an illustration showing a structural example of an opticalscanning device including elements such as an optical divider, awaveguide array, a phase shifter array, and a light source integrated ona circuit substrate (i.e., a chip);

FIG. 68 is a schematic diagram showing how two-dimensional scanning isperformed by irradiating a distant object with a light beam such as alaser beam from the optical scanning device;

FIG. 69 is a block diagram showing a structural example of a LiDARsystem that can generate a range image;

FIG. 70 is an illustration showing a schematic structure of a totalreflection waveguide;

FIG. 71 is a graph showing an electric field intensity distribution inthe total reflection waveguide;

FIG. 72 is an illustration showing a schematic structure of a slow lightwaveguide; and

FIG. 73 is a graph showing an electric field intensity distribution inthe slow light waveguide.

DETAILED DESCRIPTION

Before embodiments of the present disclosure are described, findingsunderlying the present disclosure will be described.

The present inventors have found that a problem with conventionaloptical scanning devices is that it is difficult to optically scan aspace without increasing the complexity of the structures of thedevices.

For example, in the technique disclosed in International Publication No.WO2013/168266, the driving unit for rotating the mirror is necessary.Therefore, the device structure is complicated. A problem with thisdevice is that the device is not robust against vibration.

In the optical phased array described in Japanese Unexamined PatentApplication Publication (Translation of PCT Application) No.2016-508235, light must be split and introduced into a plurality of rowwaveguides and a plurality of column waveguides to guide the split lightbeams to the plurality of antenna elements arranged in two dimensions.Therefore, wiring lines for the waveguides for guiding the light beamsare very complicated. Moreover, the range of two-dimensional scanningcannot be increased. To change the amplitude distribution of the emittedlight two dimensionally in a far field, the phase shifters must beconnected to the plurality of antenna elements arranged in twodimensions, and wiring lines for phase control must be attached to thephase shifters. The phases of the light beams entering the plurality oftwo-dimensionally arranged antenna elements can thereby be changed bydifferent amounts. Therefore, the structure of the elements is verycomplicated.

In the structure in Japanese Unexamined Patent Application PublicationNo. 2013-16591, by changing the wavelength of light entering the lightdeflection element, a large area can be scanned one-dimensionally withthe emitted light. However, a mechanism for changing the wavelength ofthe light entering the light deflection element is necessary. When sucha mechanism is installed in the light source such as a laser, a problemarises in that the structure of the light source becomes complicated.

The present inventors have focused attention on the problems in theconventional techniques and have conducted studies to solve theseproblems. The present inventors have found that the above problems canbe solved by using a waveguide element including a pair of opposedmirrors and an optical waveguide layer sandwiched between these mirrors.One of the pair of mirrors of the waveguide element has a higher lighttransmittance than the other and allows part of light propagatingthrough the optical waveguide layer to be emitted to the outside. Thedirection of the emitted light (or its emission angle) can be changed byadjusting the refractive index and/or thickness of the optical waveguidelayer, as described later. More specifically, by changing the refractiveindex and/or the thickness, a component of the wave vector of theemitted light can be changed. The component is along a directionintersecting the lengthwise direction of the optical waveguide layer.One-dimensional scanning is thereby achieved.

When an array of a plurality of waveguide elements is used,two-dimensional scanning can be achieved. More specifically, light beamswith appropriate phase differences are supplied to the plurality ofwaveguide elements, and the phase differences are controlled to change adirection in which light beams emitted from the plurality of waveguideelements are reinforced. By changing the phase differences, a componentof the wave vector of the emitted light is changed. The component isalong a direction intersecting the lengthwise direction of the opticalwaveguide layer. Two-dimensional scanning can thereby be achieved. Whentwo-dimensional scanning is performed, it is unnecessary to change therefractive indexes or thicknesses, or both, of the plurality of opticalwaveguide layers by different amounts. Specifically, two-dimensionalscanning can be performed by supplying light beams with appropriatephase differences to the plurality of optical waveguide layers andchanging the refractive indexes or thicknesses, or both, of theplurality of optical waveguide layers by the same amount in asynchronous manner. As described above, in the above embodiment of thepresent disclosure, two-dimensional optical scanning can be achievedusing the relatively simple structure.

The above-described basic principle is applicable not only to theapplication in which light is emitted but also to an application inwhich a light signal is received. By changing at least one of therefractive index and thickness (i.e., the refractive index and/or thethickness) of an optical waveguide layer, a light-receivable directioncan be changed one-dimensionally. Moreover, the light-receivabledirection can be changed two-dimensionally by changing phase differencesbetween light beams using a plurality of phase shifters connected to aplurality of waveguide elements arranged in one direction.

An optical scanning device and a photoreceiver device in embodiments ofthe present disclosure can be used for, for example, antennas of a LiDAR(Light Detection and Ranging) system. The LiDAR system useselectromagnetic waves (visible light, infrared light, or ultravioletlight) having shorter wavelengths than radio waves such as millimeterwaves used in a radar system and can therefore detect a distancedistribution of an object with high resolution. Such a LiDAR system ismounted on a mobile unit such as an automobile, a UAV (Unmanned AerialVehicle, a so-called drone), or an AGV (Automated Guided Vehicle) andused as one of crash avoidance techniques.

<Structural Example of Optical Scanning Device>

The structure of an optical scanning device for two-dimensional scanningwill be described as an example.

FIG. 1 is a perspective view schematically showing the structure of anoptical scanning device 100 in an exemplary embodiment of the presentdisclosure. The optical scanning device 100 includes a waveguide arrayincluding a plurality of waveguide elements 10 regularly arranged in afirst direction (the Y direction in FIG. 1). Each of the plurality ofwaveguide elements 10 has a shape elongated in a second direction (the Xdirection in FIG. 1) that intersects the first direction. Each of theplurality of waveguide elements 10 propagates light in the seconddirection and allows the light to be emitted in a third direction D3that intersects a plane formed by the first and second directions.Specifically, the plane formed by the first and second directions is avirtual plane parallel to the first and second directions. In thepresent embodiment, the first direction (the Y direction) and the seconddirection (the X direction) are orthogonal to each other but may not beorthogonal to each other. In the present embodiment, the plurality ofwaveguide elements 10 are arranged in the Y direction at regularintervals but are not necessarily arranged at regular intervals.

The orientation of each of structures shown in the drawings of thepresent disclosure is set in consideration of the ease of understandingof description, and the orientation of a structure when an embodiment ofthe present disclosure is actually implemented is not limited thereto.The shape and size of part or all of any of the structures shown in thedrawings do not limit the actual shape and size.

Each of the plurality of waveguide elements 10 includes a first mirror30 and a second mirror 40 (hereinafter may be referred to simply asmirrors) that face each other and further includes an optical waveguidelayer 20 located between the mirrors 30 and 40. Each of the mirrors 30and 40 has a reflecting surface that intersects the third direction D3and is located at an interface with the optical waveguide layer 20. Eachof the mirrors 30 and 40 and the optical waveguide layer 20 has a shapeelongated in the second direction (the X direction).

As described later, the first mirrors 30 of the plurality of waveguideelements 10 may be a plurality of portions of an integrally formed thirdmirror. The second mirrors 40 of the plurality of waveguide elements 10may be a plurality of portions of an integrally formed fourth mirror.The optical waveguide layers 20 of the plurality of waveguide elements10 may be a plurality of portions of an integrally formed opticalwaveguide layer. A plurality of waveguides can be formed when at leastone of the following conditions is met: (1) Each of the first mirrors 30is formed separately from the other first mirrors 30. (2) Each of thesecond mirrors 40 is formed separately from the other second mirrors 40.(3) Each of the optical waveguide layers 20 is formed separately fromthe other optical waveguide layers. The phrase “each of the firstmirrors is formed separately from the other first mirrors” means notonly that physical spaces are provided between the first mirrors butalso that a material having a different refractive index is disposedbetween the first mirrors to separate them from each other.

The reflecting surface of each first mirror 30 and the reflectingsurface of a corresponding second mirror 40 are approximately parallelto each other and face each other. Among the two mirrors 30 and 40, atleast the first mirror 30 has the capability of allowing part of lightpropagating in the optical waveguide layer 20 to pass through. In otherwords, the first mirror 30 has a higher transmittance of the above lightthan the second mirror 40. Therefore, part of the light propagating inthe optical waveguide layer 20 is emitted to the outside through thefirst mirror 30. Each of the above-described mirrors 30 and 40 may be,for example, a multilayer film mirror formed from a multilayer film (maybe referred to as a “multilayer reflective film”) made of a dielectricmaterial.

By controlling the phases of light beams inputted to the waveguideelements 10 and changing the refractive indexes or thicknesses, or both,of the optical waveguide layers 20 of the waveguide elements 10 in asynchronous manner (simultaneously), two-dimensional optical scanningcan be achieved.

To implement the above two-dimensional scanning, the present inventorshave analyzed the details of the operating principle of the waveguideelements 10. Based on the results obtained, the inventors have succeededin implementing two-dimensional optical scanning by driving theplurality of waveguide elements 10 in a synchronous manner.

As shown in FIG. 1, when light is inputted to each waveguide element 10,the light is emitted from the emission surface of the waveguide element10. The emission surface is located opposite to the reflecting surfaceof the first mirror 30. The direction D3 of the emitted light depends onthe refractive index and thickness of the optical waveguide layer andthe wavelength of the light. In the present embodiment, the refractiveindexes or thicknesses, or both, of the optical waveguide layers arecontrolled in a synchronous manner such that light beams are emittedfrom the waveguide elements 10 in approximately the same direction. Inthis manner, the X direction component of the wave vector of the lightemitted from the plurality of waveguide elements 10 can be changed. Inother words, the direction D3 of the emitted light can be changed in adirection 101 shown in FIG. 1.

Since the light beams emitted from the plurality of waveguide elements10 are directed in the same direction, the emitted light beams interferewith each other. By controlling the phases of the light beams emittedfrom the waveguide elements 10, the direction in which the light beamsare reinforced by interference can be changed. For example, when aplurality of waveguide elements 10 having the same size are arranged atregular intervals in the Y direction, light beams having differentphases shifted by a given amount are inputted to the plurality ofwaveguide elements 10. By changing the phase differences, the Ydirection component of the wave vector of the emitted light can bechanged. In other words, by changing the phase differences between thelight beams introduced into the plurality of waveguide elements 10, thedirection D3 in which the emitted light beams are reinforced byinterference can be changed in a direction 102 shown in FIG. 1.Two-dimensional optical scanning can thereby be achieved.

The operating principle of the optical scanning device 100 will next bedescribed in more detail.

<Operating Principle of Waveguide Element>

FIG. 2 is an illustration schematically showing an example of across-sectional structure of one waveguide element 10 and lightpropagating therethrough. In FIG. 2, a direction perpendicular to the Xand Y directions shown in FIG. 1 is referred to as the Z direction, anda cross section of the waveguide element 10 parallel to the XZ plane isschematically shown. In the waveguide element 10, a pair of mirrors 30and 40 are disposed so as to sandwich an optical waveguide layer 20therebetween. Light 20 introduced from one X direction end of theoptical waveguide layer 20 propagates through the optical waveguidelayer 20 while repeatedly reflected from the first mirror 30 disposed onthe upper surface of the optical waveguide layer 20 (the upper surfacein FIG. 2) and the second mirror 40 disposed on the lower surface (thelower surface in FIG. 2). The light transmittance of the first mirror 30is higher than the light transmittance of the second mirror 40.Therefore, part of the light can be outputted mainly from the firstmirror 30.

In an ordinary waveguide such as an optical fiber, light propagatesthrough the waveguide while undergoing total reflection repeatedly.However, in the waveguide element 10 in the present embodiment, lightpropagates while repeatedly reflected from the mirrors 30 and 40disposed on the upper and lower surfaces, respectively, of the opticalwaveguide layer 20. Therefore, there is no constraint on the propagationangle of the light (the incident angle at the interface between theoptical waveguide layer 20 and the mirror 30 or 40), and light incidenton the mirror 30 or 40 at an angle closer to the vertical is allowed topropagate. Specifically, light incident on the interface at an anglesmaller than the critical angle of total reflection (i.e., an anglecloser to the vertical) can be propagated. Therefore, the propagationvelocity (group velocity) of light in its propagation direction is muchlower that the velocity of light in free space. Thus, the waveguideelement 10 has such characteristics that the propagation conditions oflight are largely changed according to changes in the wavelength of thelight, the thickness of the optical waveguide layer 20, and therefractive index of the optical waveguide layer 20.

The propagation of light through the waveguide element 10 will bedescribed in more detail. Let the refractive index of the opticalwaveguide layer 20 be n_(w), and the thickness of the optical waveguidelayer 20 be d. The thickness d of the optical waveguide layer 20 is thesize of the optical waveguide layer 20 in the direction normal to thereflecting surface of the mirror 30 or 40. In consideration of lightinterference conditions, the propagation angle θ_(w) of light with awavelength λ satisfies formula (1) below.

2dn _(w) cos θ_(w) =mλ  (1)

Here, m is the mode order. Formula (1) corresponds to a condition forallowing the light to form a standing wave in the optical waveguidelayer 20. When the wavelength λ_(g) in the optical waveguide layer 20 isλ/n_(w), the wavelength λ_(g)′ in the thickness direction of the opticalwaveguide layer 20 is considered to be λ/(n_(w) cos θ_(w)). When thethickness d of the optical waveguide layer 20 is equal to an integermultiple of one half of the wavelength λ_(g)′ in the thickness directionof the optical waveguide layer 20, i.e., λ/(2n_(w) cos θ_(w)), astanding wave is formed. Formula (1) is obtained from this condition. min formula (1) represents the number of loops (anti-nodes) of thestanding wave.

When the mirrors 30 and 40 are multilayer film mirrors, light penetratesinto the mirrors at the time of reflection. Therefore, strictlyspeaking, a term corresponding to the penetration path length of thelight must be added to the left-hand side of formula (1). However, sincethe influences of the refractive index n_(w) and thickness d of theoptical waveguide layer 20 are much larger than the influence of thelight penetrating into the mirrors, the fundamental behavior of thelight can be explained by formula (1).

The emission angle θ when the light propagating through the opticalwaveguide layer 20 is emitted to the outside (typically the air) throughthe first mirror 30 can be denoted by formula (2) below according to theSnell's law.

sin θ=n _(w) sin θ_(w)  (2)

Formula (2) is obtained from the condition that, on the light emissionsurface, the wavelength λ/sin θ of the light in a surface direction onthe air side is equal to the wavelength λ/(n_(w) sin θ_(w)) of the lightin the propagation direction on the waveguide element 10 side.

From formulas (1) and (2), the emission angle θ can be denoted byformula (3) below.

$\begin{matrix}{{\sin \; \theta} = \sqrt{n_{w}^{2} - \left( \frac{m\; \lambda}{2d} \right)^{2}}} & (3)\end{matrix}$

As can be seen from formula (3), by changing the wavelength λ of thelight, the refractive index n_(w) of the optical waveguide layer 20, orthe thickness d of the optical waveguide layer 20, the emissiondirection of the light can be changed.

For example, when n_(w)=2, d=387 nm, λ=1,550 nm, and m=1, the emissionangle is 0°. When the refractive index n_(w) is changed from the abovestate to 2.2, the emission angle is changed to about 66°. When thethickness d is changed to 420 nm while the refractive index isunchanged, the emission angle is changed to about 51°. When thewavelength λ is changed to 1,500 nm while the refractive index and thethickness are unchanged, the emission angle is changed to about 30°. Asdescribed above, the emission direction of the light can be largelychanged by changing the wavelength λ of the light, the refractive indexn_(w) of the optical waveguide layer 20, or the thickness d of theoptical waveguide layer 20.

To control the emission direction of the light by utilizing the aboveprinciple, it is contemplated to provide a wavelength changing mechanismthat changes the wavelength of the light propagating through the opticalwaveguide layer 20. However, when the wavelength changing mechanism isinstalled in a light source such as a laser, the structure of the lightsource becomes complicated.

In the optical scanning device 100 in the present embodiment, theemission direction of light is controlled by controlling one or both ofthe refractive index n_(w) and thickness d of the optical waveguidelayer 20. In the present embodiment, the wavelength λ of the light isunchanged during operation and held constant. No particular limitationis imposed on the wavelength λ. For example, the wavelength λ may bewithin the wavelength range of 400 nm to 1,100 nm (the visible toinfrared range) in which high detection sensitivity can be obtained byusing one of a general photo detector and a general image sensor thatdetect light through light absorption by silicon (Si). In anotherexample, the wavelength λ may be within the near-infrared range of 1,260nm to 1,625 nm in which transmission loss in an optical fiber or a Siwaveguide is relatively small. However, the above wavelength ranges aremerely examples. The wavelength range of the light used is not limitedto the visible or infrared wavelength range and may be, for example, anultraviolet wavelength range. In the present embodiment, the wavelengthis not controlled. However, in addition to the control of the refractiveindex and/or the thickness, the wavelength may be changed andcontrolled.

The present inventors have examined by optical analysis whether lightcan be actually emitted in a specific direction as described above. Theoptical analysis was performed by computation using DiffractMODavailable from Cybernet Systems Co., Ltd. This is a simulation based onrigorous coupled-wave analysis (RCWA), and the effects of wave opticscan be correctly computed.

FIG. 3 is an illustration schematically showing a computational modelused for the simulation. In this computational model, a second mirror40, an optical waveguide layer 20, and a first mirror 30 are stacked inthis order on a substrate 50. Each of the first mirror 30 and the secondmirror 40 is a multilayer film mirror including a dielectric multilayerfilm. The second mirror 40 has a structure in which six low-refractiveindex layers 42 having a lower refractive index and six high-refractiveindex layers 44 having a higher refractive index (a total of twelvelayers) are alternately stacked. The first mirror 30 has a structure inwhich two low-refractive index layers 42 and two high-refractive indexlayers 44 (a total of four layers) are alternately stacked. The opticalwaveguide layer 20 is disposed between the mirrors 30 and 40. A mediumother than the waveguide element 10 and the substrate 50 is air.

The optical response to incident light was examined using the abovemodel while the incident angle of the light was changed. Thiscorresponds to examination of the degree of coupling of the incidentlight from air into the optical waveguide layer 20. Under the conditionthat the incident light is coupled into the optical waveguide layer 20,the reverse process occurs in which the light propagating through theoptical waveguide layer 20 is emitted to the outside. Therefore, thedetermination of the incident angle when the incident light is coupledinto the optical waveguide layer 20 corresponds to the determination ofthe emission angle when the light propagating through the opticalwaveguide layer 20 is emitted to the outside. When the incident light iscoupled into the optical waveguide layer 20, light loss occurs in theoptical waveguide layer 20 due to absorption and scattering of thelight. Specifically, under the condition that a large loss occurs, theincident light is strongly coupled into the optical waveguide layer 20.When there is no light loss due to absorption, etc., the sum of thelight transmittance and reflectance is 1. However, when there is a loss,the sum of the transmittance and reflectance is less than 1. In thiscomputation, to take the influence of light absorption intoconsideration, an imaginary part was added to the refractive index ofthe optical waveguide layer 20, and a value obtained by subtracting thesum of the transmittance and reflectance from 1 was used as themagnitude of the loss.

In this simulation, the substrate 50 is Si, the low-refractive indexlayers 42 are SiO₂ (thickness: 267 nm), and the high-refractive indexlayers 44 are Si (thickness: 108 nm). The magnitude of loss was computedwhile the incident angle of light with a wavelength λ=1.55 μm waschanged.

FIG. 4A shows the results of the computations of the relation betweenthe refractive index n_(w) of the optical waveguide layer 20 and theemission angle θ of light with a mode order of m=1 when the thickness dof the optical waveguide layer 20 is 704 nm. White lines indicate thatthe loss is large. As shown in FIG. 4A, the emission angle θ of thelight with a mode order of m=1 is 0° near n_(w)=2.2. One example of amaterial having a refractive index n_(w) of around 2.2 is lithiumniobate.

FIG. 4B shows the results of the computations of the relation betweenthe refractive index n_(w) of the optical waveguide layer 20 and theemission angle θ of light with a mode order of m=1 when the thickness dof the optical waveguide layer 20 is 446 nm. As shown in FIG. 4B, theemission angle θ of the light with a mode order of m=1 is 0° nearn_(w)=3.45. One example of a material having a refractive index n_(w) ofaround 3.45 is silicon (Si).

As described above, the waveguide element 10 can be designed such that,when the optical waveguide layer 20 has a specific refractive indexn_(w), the emission angle θ of light with a specific mode order (e.g.,m=1) is set to be 0° by adjusting the thickness d of the opticalwaveguide layer 20.

As can be seen from FIGS. 4A and 4B, the emission angle θ is largelychanged according to the change in the refractive index. As describedlater, the refractive index can be changed by various methods such ascarrier injection, an electro-optical effect, and a thermo-opticaleffect. However, the change in the refractive index by such a method isnot so large, i.e., about 0.1. Therefore, it has been considered thatsuch a small change in refractive index does not cause a large change inthe emission angle. However, as can be seen from FIGS. 4A and 4B, nearthe refractive index at which the emission angle θ is 0°, when therefractive index increases by 0.1, the emission angle θ is changed from0° to about 30°. As described above, in the waveguide element 10 in thepresent embodiment, even a small change in the refractive index cancause the emission angle to be changed largely.

Similarly, as can be seen from comparison between FIGS. 4A and 4B, theemission angle θ changes largely according to the change in thethickness d of the optical waveguide layer 20. As described later, thethickness d can be changed using, for example, an actuator connected toat least one of the two mirrors. Even when the change in the refractiveindex is small, the emission angle can be largely changed.

As described above, by changing the refractive index n_(w) of theoptical waveguide layer 20 and/or its thickness d, the direction of thelight emitted from the waveguide element 10 can be changed. To achievethis, the optical scanning device 100 in the present embodiment includesa first adjusting element that changes at least one of the refractiveindex and thickness of the optical waveguide layer 20 of each of thewaveguide elements 10. A structural example of the first adjustingelements will be described later.

As described above, the use of the waveguide element 10 allows theemission direction of light to be changed largely by changing at leastone of the refractive index n_(w) and thickness d of the opticalwaveguide layer 20. In this manner, the emission angle of the lightemitted from the mirror 30 can be changed in a direction along thewaveguide element 10. To achieve the above-described one-dimensionalscanning, it is unnecessary to use an array of waveguide elements 10,and it is only necessary to use at least one waveguide element 10.

FIG. 5 is an illustration schematically showing an example of theoptical scanning device 100 that can implement one-dimensional scanningusing a single waveguide element 10. In this example, a beam spotextending in the Y direction is formed. By changing the refractive indexof the optical waveguide layer 20, the beam spot can be moved in the Xdirection. One-dimensional scanning can thereby be achieved. Since thebeam spot extends in the Y direction, a relatively large area extendingtwo-dimensionally can be scanned by uniaxial scanning. The structureshown in FIG. 5 may be employed in applications in which two-dimensionalscanning is unnecessary.

To implement two-dimensional scanning, the waveguide array in which theplurality of waveguide elements 10 are arranged is used, as shown inFIG. 1. When the phases of light beams propagating through the pluralityof waveguide elements 10 satisfy a specific condition, the light beamsare emitted in a specific direction. When the condition for the phasesis changed, the emission direction of the light beams is changed also inthe arrangement direction of the waveguide array. Specifically, the useof the waveguide array allows two-dimensional scanning to beimplemented. An example of a specific structure for implementing thetwo-dimensional scanning will be described later.

As described above, when at least one waveguide element 10 is used, theemission direction of light can be changed by changing at least one ofthe refractive index and thickness of the optical waveguide layer 20 ofthe waveguide element 10. However, there is a room for improvement inthe structure for efficiently introducing light into the waveguideelement 10. Unlike a general waveguide that uses total reflection oflight (hereinafter may be referred to as a “total reflectionwaveguide”), the waveguide element 10 in the present embodiment in thepresent disclosure has the waveguide structure in which the opticalwaveguide layer is sandwiched between the pair of mirrors (e.g.,multilayer reflective films) (this structure may be hereinafter referredto as a “reflective waveguide”). Coupling of light into such areflective waveguide has not been studied sufficiently. The presentinventors have devised a novel structure for efficiently introducinglight into the optical waveguide layer 20.

FIG. 6A is a cross-sectional view schematically showing an example(comparative example) of a structure in which light is indirectlyinputted into the optical waveguide layer 20 through air and the mirror30. In this comparative example, the propagating light is indirectlyintroduced from the outside through air and the mirror 30 into theoptical waveguide layer 20 of the waveguide element 10, which is areflective waveguide. To introduce the light into the optical waveguidelayer 20, the reflection angle θ_(w) of the guided light inside theoptical waveguide layer 20 must satisfy the Snell's law (n_(in) sinθ_(in)=n_(w) sin θ_(w)). Here, n_(in) is the refractive index of theexternal medium, θ_(in) is the incident angle of the propagating light,and n_(w) is the refractive index of the optical waveguide layer 20. Byadjusting the incident angle θ_(in) in consideration of the abovecondition, the coupling efficiency of the light can be maximized. Inthis example, the number of films in the multilayer reflective film issmaller in a portion of the first mirror 30 than in the other portion.The light is inputted from this portion, and the coupling efficiency canthereby be increased. However, in the above structure, the incidentangle θ_(in) of the light on the optical waveguide layer 20 must bechanged according to the change in the propagation constant of theoptical waveguide layer 20 (the change in θ_(wav)).

One method to maintain the state in which the light can be alwayscoupled into the waveguide even when the propagation constant of theoptical waveguide layer 20 is changed is to cause a diverging beam to beincident on the portion of the multilayer reflective film that includesa reduced number of films. In one example of such a method, an opticalfiber 7 inclined at an angle θ_(in) with respect to the direction normalto the mirror 30 is used to cause light to enter the waveguide element10 from the outside indirectly through air and the mirror 30, as shownin FIG. 6B. The coupling efficiency in this case will be examined. Forthe sake of simplicity, the light is assumed to be a ray of light. Thenumerical aperture (NA) of an ordinary single mode fiber is about 0.14.This corresponds to an angle of about ±8 degrees. The range of theincident angle of the light coupled into the waveguide is comparable tothe divergence angle of the light emitted from the waveguide. Thedivergence angle θ_(div) of the emitted light is represented by formula(4) below.

$\begin{matrix}{\theta_{div} \approx \frac{\lambda}{L\; \cos \; \theta_{out}}} & (4)\end{matrix}$

Here, L is a propagation length, λ is the wavelength of the light, andθ_(out) is the emergent angle of the light. When L is 10 μm or more,θ_(div) is at most 1 degree or less. Therefore, the coupling efficiencyof the light from the optical fiber 7 is 1/16≈100≈6.3% or less. FIG. 7shows the results of computations of changes in the coupling efficiencywhen the refractive index n_(w) of the waveguide was changed to changethe emergent angle θ_(out) of the light while the incident angle θ_(in)of the light was fixed. The coupling efficiency is the ratio of theenergy of the guided light to the energy of the incident light. Theresults shown in FIG. 7 were obtained by computing the couplingefficiency using an incident angle θ_(in) of 30°, a waveguide thicknessof 1.125 μm, and a wavelength of 1.55 μm. In the above computations, therefractive index n_(w) was changed within the range of 1.44 to 1.78 tochange the emergent angle θ_(out) within the range of 10° to 65°. Asshow in FIG. 7, in this structure, the coupling efficiency is at mostless than 7%. When the emergent angle θ_(out) is changed by 20° or morefrom the emergent angle that gives the maximum coupling efficiency, thecoupling efficiency is reduced to one-half or less of the maximumcoupling efficiency.

As described above, when the propagation constant is changed bychanging, for example, the refractive index of the waveguide in order toperform optical scanning, the coupling efficiency is further reduced. Tomaintain the coupling efficiency, it is also necessary to change theincident angle θ_(in) of the light according to the change in thepropagation constant. However, introduction of a mechanism for changingthe incident angle θ_(in) of the light is not desirable because thedevice structure becomes complicated. The present inventors have foundthat the light incident angle can be fixed when a region including awaveguide whose refractive index and thickness are maintained constantis provided upstream of a region including a waveguide whose refractiveindex or thickness is to be changed.

There are two important factors for coupling of guided light between twodifferent waveguides. One of them is the propagation constant of thepropagating light, and the other one is the electric field intensitydistribution of each mode. The closer the propagation constant and theelectric field intensity distribution in one of the two waveguides areto those in the other, the higher the coupling efficiency. Thepropagation constant β of light propagating through a waveguide isrepresented by β=k·sin θ_(w)=(λπn_(w) sin θ_(w))/λ when the light istreated in a geometrical optics manner for simplicity. Here, k is thewave number, θ_(w) is the angle of the guided light, and n_(w) is therefractive index of the waveguide layer. In a total reflectionwaveguide, the guided light is confined in the waveguide layer byutilizing total reflection, so that the total reflection condition n_(w)sin θ_(w)>1 is satisfied. However, in a slow light waveguide, light isconfined in the waveguide by using multilayer reflective films presentabove and below the waveguide, and part of the guided light is emittedthrough the multilayer reflective films, so that n_(w) sin θ_(w)<1. Thepropagation constant in the total reflection waveguide cannot be thesame as the propagation constant in the slow light waveguide from whichpart of the guided light is emitted. The electric field intensitydistribution in a total reflection waveguide shown in FIG. 70 has a peakwithin the waveguide as shown in FIG. 71, and the electric fieldintensity decreases monotonically outside the waveguide. In a slow lightwaveguide shown in FIG. 72, the electric field intensity distribution isas shown in FIG. 73. The electric field intensity distribution has apeak within the waveguide, as in the above case. However, the guidedlight is reflected in the dielectric multilayer films due tointerference. Therefore, as shown in FIG. 73, the electric fieldintensity penetrates deep into the dielectric multilayer films andvaries in a vibrating manner. As described above, the propagationconstant of the guided light and the electric field intensitydistribution in the total reflection waveguide differ largely from thosein the slow light waveguide. Therefore, it has not been contemplated toconnect a total reflection waveguide directly to a slow light waveguide.The present inventors have found that a total reflection waveguide canbe connected directly to an optical waveguide layer having a variablerefractive index and/or a variable thickness.

The present inventors have also found that, by disposing these two typesof waveguides on a common substrate, an optical scanning device can beproduced easily. Specifically, the two types of waveguides may bedisposed on a single integrally formed substrate. A general waveguide isproduced on a substrate using a semiconductor process. The structure ofthe waveguide is generally formed on the substrate using, for example, acombination of deposition by vacuum evaporation, sputtering, etc. andfine patterning by lithography, etching, etc. Examples of the materialof the substrate include Si, SiO₂, GaAs, and GaN.

A reflective waveguide can be produced using a similar semiconductorprocess. In the reflective waveguide, one of a pair of mirrorssandwiching an optical waveguide layer allows light to pass through, andthe light is thereby emitted. In most cases, the mirrors are formed on aglass substrate available at low cost. A substrate made of Si, SiO₂,GaAs, GaN, etc. may be used instead of the glass substrate.

By connecting a reflective waveguide to another waveguide, light can beintroduced into the reflective waveguide.

FIG. 8 is an illustration schematically showing connections between aplurality of first waveguides 1 produced on a substrate 50A and aplurality of second waveguides 10 produced on another substrate 50B. Thetwo substrates 50A and 50B are disposed parallel to each other in the XYplane. The plurality of first waveguides 1 and the plurality of secondwaveguides 10 extend in the X direction and are arranged in the Ydirection. The first waveguides 1 are, for example, general waveguidesthat use total reflection of light. The second waveguides 10 arereflective waveguides. The first waveguides 1 and the second waveguides10 disposed on the different substrates 50A and 50B, respectively, arealigned and connected with each other, and this allows light to beintroduced from the first waveguides 1 into the second waveguides 10.

To introduce light from the first waveguides 1 into the secondwaveguides 10 efficiently, it is desired that the waveguides are alignedwith very high precision on the order of 10 nm. Even when the waveguidesare aligned with high precision, if the thermal expansion coefficientsof the two substrates 50A and 50B differ from each other, the alignmentmay be changed due to a change in temperature. For example, the thermalexpansion coefficients of Si, SiO₂, GaAs, and GaN are about 4, 0.5, 6,and 5 (×10⁻⁶/K), respectively, and the thermal expansion coefficient ofBK7, which is often used for a glass substrate, is 9 (×10⁻⁶/K). Evenwhen any two of these materials are used for the above substrates, thedifference in thermal expansion coefficient is 1×10⁻⁶/K or more. Forexample, when the size of the substrates 50A and 50B in the arrangementdirection of the plurality of first waveguides 1 and the plurality ofsecond waveguides 10 (in the Y direction in FIG. 8) is 1 mm, atemperature change of 1° C. causes the alignment between the twosubstrates 50A and 50B to be changed by 1 nm. A temperature change ofseveral tens of degrees Celsius causes the alignment between the twosubstrates 50A and 50B to be largely changed by several tens to severalhundreds of nanometers. Therefore, light cannot be efficientlyintroduced from the first waveguides 1 into the second waveguides 10.

The present inventors have found that the above problem can be solved bydisposing the first waveguides and the second waveguides on the samesubstrate. When these waveguides are disposed on the common substrate,the first waveguides and the second waveguides can be easily alignedwith each other. Moreover, a change in the alignment between the firstwaveguides and the second waveguides due to thermal expansion can beprevented. Therefore, light can be efficiently introduced from the firstwaveguides into the second waveguides.

An optical scanning device in one embodiment of the present disclosureincludes a first waveguide, a second waveguide connected to the firstwaveguide, and a substrate that supports the first and secondwaveguides. The second waveguide includes a first mirror having amultilayer reflective film, a second mirror having a multilayerreflective film facing the multilayer reflective film of the firstmirror, and an optical waveguide layer that is located between the firstmirror and the second mirror and propagates light inputted to the firstwaveguide and transmitted through the first waveguide. The first mirrorhas a higher light transmittance than the second mirror and allows partof the light propagating through the optical waveguide layer to beemitted to the outside of the optical waveguide layer. The opticalscanning device further includes an adjusting element that changes atleast one of the refractive index and thickness of the optical waveguidelayer to thereby change the direction of the emitted light.

In the present embodiment, the “second waveguide” corresponds to the“waveguide element” in the preceding embodiment. In the presentembodiment of the present disclosure, the first waveguide whoserefractive index and thickness are maintained constant is disposedupstream of the second waveguide, and light is inputted to the firstwaveguide. The first waveguide propagates the inputted light, and thelight is inputted to the second waveguide from its end surface. An endsurface of the first waveguide may be connected directly to the endsurface of the second waveguide, or, for example, a gap may be providedbetween these end surfaces. In the present specification, the phrase“the first waveguide is connected to the second waveguide” means thatthe first waveguide and the second waveguide are positioned such thatlight can be transferred between them. The form of “connection betweenthe first waveguide and the second waveguide” includes not only the formin which the first waveguide is connected directly to the secondwaveguide (i.e., they are in contact with each other) but also the formin which they are disposed through a gap sufficiently shorter than thewavelength of the propagating light. In the present disclosure, thephrase “A is connected directly to B” means that a portion of A and aportion of B are in contact with each other such that light can betransferred between A and B.

In the above structure, since the first waveguide is disposed upstreamof the second waveguide (waveguide element), a reduction in couplingefficiency due to scanning (i.e., loss of energy) can be suppressed evenwhen the incident angle of light incident on the first waveguide is heldconstant.

In the above structure, since the first and second waveguides aredisposed on the same substrate, the first and second waveguides areeasily aligned with each other. Moreover, a change in the alignmentbetween the first and second waveguides due to thermal expansion can besuppressed. Therefore, light can be efficiently introduced from thefirst waveguide into the second waveguide.

A third waveguide may be disposed upstream of the first waveguide. Thethird waveguide is connected to the first waveguide and allows lighttransmitted through the third waveguide to be inputted to the firstwaveguide. In one embodiment, the third waveguide may be a totalreflection waveguide, and the second waveguide may be a reflectivewaveguide. The substrate that supports the first and second waveguidesmay further support the third waveguide.

FIG. 9 is a cross-sectional view of a waveguide element 10 in the YZplane, schematically showing a structural example in which spacers 73are disposed on both sides of an optical waveguide layer 20 locatedbetween a first mirror 30 and a second mirror 40. The refractive indexn_(low) of the spacers 73 is lower than the refractive index n_(w) ofthe optical waveguide layer (n_(low)<n_(w)). The spacers 73 may be, forexample, air. The spacers 73 may be, for example, TiO₂, Ta₂O₅, SiN, AlN,SiO₂, etc., so long as the spacers 73 have a lower refractive index thanthe optical waveguide layer.

FIG. 10 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing a structural example of a waveguidearray 10A in which the waveguide elements 10 in FIG. 9 are arranged inthe Y direction. In the structural example in FIG. 10, the width of thefirst mirrors 30 in the Y direction is the same as the width of theoptical waveguide layers 20. The leak of guided light from regions inwhich no first mirror 30 is present is reduced if the width of the firstmirror 30 is longer than the width of the optical waveguide layers 20.In an array of a plurality of waveguide elements 10 including aplurality of reflective waveguides, leakage of guided light can beprevented when at least one of the width of first mirrors 30 and thewidth of second mirrors 40 is larger than the width of optical waveguidelayers 20. However, such an idea has not been employed previously.

To improve light scanning performance, it is desirable to reduce thewidth of each of the waveguide elements 10 of the waveguide array 10A.However, in this case, the guided light leakage problem becomes moreprominent.

The reason for the leakage of guided light will be described.

FIG. 11 is an illustration schematically showing propagation of guidedlight in the X direction within an optical waveguide layer 20. Sincen_(w)>n_(low), the guided light is confined by total reflection in the±Y directions and propagates in the X direction. However, in practice,evanescent light leaks out from the Y direction edge surfaces of theoptical waveguide layer 20. As shown in FIG. 2, the guided lightpropagates in the X direction at an angle smaller than the totalreflection angle θ_(in) while reflected by the first and second mirrors30 and 40 in the ±Z directions. In this case, in the regions with nofirst mirror 30 shown in FIG. 10, the evanescent light is not reflectedand leaks to the outside. This unintended light loss may cause theamount of light used for optical scanning to be reduced.

The present inventors have found that the above problem can be solved bysetting at least one of the width of the first mirrors 30 in thearrangement direction of the plurality of waveguide elements 10 and thewidth of the second mirrors 40 to be larger than the width of theoptical waveguide layers 20. This can reduce the unintended light lossdescribed above. Therefore, a reduction in the amount of light used foroptical scanning is prevented.

The present disclosure encompasses devices described in the followingitems.

[Item 1] An optical scanning device including:

a waveguide array including a plurality of waveguide elements that arearranged in a first direction and that propagate light in a seconddirection intersecting the first direction, the waveguide array beingconfigured to emit light from the plurality of waveguide elements in athird direction intersecting a plane formed by the first and seconddirections; and

a first adjusting element that changes the third direction in which thelight is emitted from the plurality of waveguide elements,

wherein each of the plurality of waveguide elements includes

a first mirror extending in the second direction and having a reflectingsurface that intersects the third direction,

a second mirror extending in the second direction and having areflecting surface that faces the reflecting surface of the firstmirror, and

an optical waveguide layer that is located between the first mirror andthe second mirror and that propagates a light beam in the seconddirection,

wherein, in each of the plurality of waveguide elements, at least one ofthe width of the first mirror in the first direction and the width ofthe second mirror in the first direction is larger than the width of theoptical waveguide layer in the first direction, and

the first mirror has a higher light transmittance than the second mirrorand allows part of the light beam propagating through the opticalwaveguide layer to be emitted to the outside of the optical waveguidelayer, and

wherein the first adjusting element changes at least one of therefractive index and thickness of the optical waveguide layer of each ofthe waveguide elements to thereby change the third direction in whichthe light is emitted from the plurality of waveguide elements.

[Item 2] The optical scanning device according to item 1,

wherein, in each of the plurality of waveguide elements, at least one ofthe first and second mirrors has a protruding portion that protrudes inthe first direction from an edge surface of the optical waveguide layer,and

y₁≥7y_(d) holds,

where y₁ is the size of the protruding portion in the first direction,and

y_(d) is the distance in the first direction between the edge surface ofthe optical waveguide layer and a position at which the light intensityof evanescent light from the optical waveguide layer is 1/e of the lightintensity of the evanescent light from the optical waveguide layer atthe edge surface of the optical waveguide layer,

where e is the base of natural logarithm.

[Item 3] The optical scanning device according to item 1,

wherein, in each of the plurality of waveguide elements, at least one ofthe first and second mirrors has a portion protruding in the firstdirection from an edge surface of the optical waveguide layer, and

wherein y₁≥11_(yd) holds,

where y₁ is the size of the protruding portion in the first direction,and

y_(d) is the distance in the first direction between the edge surface ofthe optical waveguide layer and a position at which the light intensityof evanescent light from the optical waveguide layer is 1/e of the lightintensity of the evanescent light from the optical waveguide layer atthe edge surface of the optical waveguide layer,

where e is the base of natural logarithm.

[Item 4] The optical scanning device according to item 1, wherein atleast one of the first and second mirrors of each of the waveguideelements is part of one integrated mirror.[Item 5] The optical scanning device according to item 1,

wherein the first mirror of each of the waveguide elements is part ofone integrated mirror, and

wherein the second mirror of each of the waveguide elements is part ofanother integrated mirror.

[Item 6] The optical scanning device according to any of items 1 to 5,

wherein a region between the optical waveguide layers of each twoadjacent waveguides of the plurality of waveguides contains a materialhaving a lower refractive index than the optical waveguide layers.

[Item 7] The optical scanning device according to any of items 1 to 6,

wherein the optical waveguide layer of each of the waveguide elementscontains a material whose refractive index for the light beampropagating through the optical waveguide layer is changed when avoltage is applied, and

wherein the first adjusting element includes a pair of electrodessandwiching one or more optical waveguide layers of the waveguideelements and changes the refractive index of each of the one or moreoptical waveguide layers by applying a voltage to the pair ofelectrodes.

[Item 8] The optical scanning device according to item 7,

wherein the optical waveguide layer of each of the waveguide elementscontains an electrooptical material, and

wherein the first adjusting element changes the refractive index of theelectrooptical material by applying a voltage to the pair of electrodes.

[Item 9] The optical scanning device according to item 7,

wherein the optical waveguide layer of each of the waveguide elementscontains a liquid crystal material, and

wherein the first adjusting element changes the refractive indexanisotropy of the liquid crystal material by applying a voltage to thepair of electrodes to thereby change the refractive index of the one ormore optical waveguide layer.

[Item 10] The optical scanning device according to item 8,

wherein a region between the optical waveguide layers of each twoadjacent waveguides of the plurality of waveguides contains SiO₂, TiO₂,Ta₂O₅, SiN, AlN, or air.

[Item 11] The optical scanning device according to item 9,

wherein the region between the optical waveguide layers of each twoadjacent waveguides of the plurality of waveguides contains SiO₂ or air.

[Item 12] The optical scanning device according to any of items 1 to 11,

wherein, when a second direction component of the wave vector of thelight emitted in the third direction is denoted as an X component and afirst direction component of the wave vector is denoted as a Ycomponent,

the first adjusting element changes at least one of the refractive indexand thickness of the optical waveguide layer of each of the waveguideelements to thereby change the X component of the wave vector, and

the Y component of the wave vector is changed when the difference inphase between light beams to be supplied to each two adjacent waveguideelements of the plurality of waveguide elements is changed.

[Item 13] The optical scanning device according to any of items 1 to 12,

wherein the first direction is orthogonal to the second direction.

[Item 14] The optical scanning device according to any of items 1 to 13,

wherein the plurality of waveguide elements are arranged in the firstdirection at regular intervals.

[Item 15] The optical scanning device according to any of items 1 to 14,

wherein, in each of the plurality of waveguide elements, at least one ofthe first and second mirrors includes a dielectric multilayer film.

[Item 16] The optical scanning device according to any of items 1 to 15,further including:

a plurality of phase shifters connected to the respective waveguideelements, each of the plurality of phase shifters including a waveguidethat is connected to the optical waveguide layer of a corresponding oneof the plurality of waveguide elements directly or through anotherwaveguide; and

a second adjusting element that changes phase differences between lightbeams propagating from the plurality of phase shifters to the pluralityof waveguide elements to thereby change the third direction in which thelight is emitted from the plurality of waveguide elements.

[Item 17] The optical scanning device according to item 16,

wherein the waveguide of each of the phase shifters contains a materialwhose refractive index is changed when a voltage is applied ortemperature is changed, and

wherein the second adjusting element changes the refractive index of thewaveguide of each of the phase shifters by applying a voltage to thewaveguide or changing the temperature of the waveguide to thereby changethe phase differences between the light beams propagating from theplurality of phase shifters to the plurality of waveguide elements.

[Item 18] A photoreceiver device including:

a waveguide array including a plurality of waveguide elements that arearranged in a first direction and propagate light in a second directionintersecting the first direction, the waveguide array being configuredto allow light entering the plurality of waveguide elements in a thirddirection intersecting both the first and second directions to propagatein the second direction; and

a first adjusting element that adjusts a light-receivable direction,

wherein each of the plurality of waveguide elements includes:

a first mirror extending in the second direction and having a reflectingsurface that intersects the third direction;

a second mirror extending in the second direction and having areflecting surface that faces the reflecting surface of the firstmirror; and

an optical waveguide layer that is located between the first mirror andthe second mirror and propagates light in the second direction,

wherein, in each of the plurality of waveguide elements, at least one ofthe width of the first mirror in the first direction and the width ofthe second mirror in the first direction is larger than the width of theoptical waveguide layer in the first direction, and

wherein the first adjusting element changes at least one of therefractive index and thickness of the optical waveguide layer of each ofthe waveguide elements to thereby change the light-receivable direction.

[Item 19] The photoreceiver device according to item 18, furtherincluding:

a plurality of phase shifters connected to the respective waveguideelements, each of the plurality of phase shifters including a waveguidethat is connected to the optical waveguide layer of a corresponding oneof the plurality of waveguide elements directly or through anotherwaveguide; and

a second adjusting element that changes phase differences between lightbeams passing through the plurality of phase shifters and outputted fromthe plurality of waveguide elements to thereby change thelight-receivable direction.

[Item 20] A LiDAR system including:

the optical scanning device according to any of items 1 to 17;

a photodetector that detects light emitted from the optical scanningdevice and reflected from a target; and

a signal processing circuit that generates distance distribution databased on an output from the photodetector.

In the present disclosure, the “light” means electromagnetic wavesincluding not only visible light (wavelength: about 400 nm to about 700nm) but also ultraviolet rays (wavelength: about 10 nm to about 400 nm)and infrared rays (wavelength: about 700 nm to about 1 mm). In thepresent specification, the ultraviolet rays may be referred to as“ultraviolet light,” and the infrared rays may be referred to as“infrared light.”

In the present disclosure, the “scanning” with light means that thedirection of the light is changed. The “one-dimensional scanning” meansthat the direction of the light is linearly changed in a directionintersecting the direction of the light. The “two-dimensional scanning”means that the direction of the light is changed two-dimensionally alonga plane intersecting the direction of the light.

Embodiments of the present disclosure will next be described morespecifically. However, unnecessarily detailed description may beomitted. For example, detailed description of well-known matters andredundant description of substantially the same structures may beomitted. This is to avoid unnecessary redundancy in the followingdescription and to facilitate understanding by those skilled in the art.The present inventors provide the accompanying drawings and thefollowing description to allow those skilled in the art to fullyunderstand the present disclosure. The accompanying drawings and thefollowing description are not intended to limit the subject matterdefined in the claims. In the following description, the same or similarcomponents are denoted by the same reference numerals.

Embodiments

FIG. 12 is a cross-sectional view schematically showing part of thestructure of an optical scanning device in an exemplary embodiment ofthe present disclosure. The optical scanning device includes a firstwaveguide 1 and a second waveguide (also referred to as waveguideelement) 10 connected to the first waveguide. The second waveguide 10includes a first mirror 30 including a multilayer reflective film, asecond mirror 40 including a multilayer reflective film facing themultilayer reflective film of the first mirror 30, and an opticalwaveguide layer 20 located between the first mirror 30 and the secondmirror 40. The optical waveguide layer 20 propagates light inputted intothe first waveguide 1 and transmitted through the first waveguide 1. Theoptical waveguide layer 20 propagates the light in the same direction asthe guiding direction of the first waveguide 1. The first mirror 30 hasa higher light transmittance than the second mirror 40 and allows partof the light propagating through the optical waveguide layer 20 to beemitted to the outside of the optical waveguide layer 20. Although notshown in FIG. 12, the optical scanning device 100 further includes anadjusting element that changes at least one of the refractive index andthickness of the optical waveguide layer 20. The optical waveguide layer20 contains a material whose refractive index for the light propagatingthrough the optical waveguide layer 20 is changed when, for example, avoltage is applied. The adjusting element changes the refractive indexof the optical waveguide layer 20 by applying a voltage to the opticalwaveguide layer 20 to thereby change the direction of the light emittedfrom the second waveguide 10.

The first waveguide 1 includes two opposed multilayer reflective films 3and 4 and an optical waveguide layer 2 sandwiched between the twomultilayer reflective films 3 and 4. To transmit the light guided by thefirst waveguide 1 with no loss, it is desirable that the multilayerreflective films 3 and 4 in the first waveguide 1 have higherreflectance (lower transmittance) than the light-emitting-sidemultilayer reflective film (the first mirror 30) of the second waveguide10. Therefore, desirably, the thicknesses of the multilayer reflectivefilms 3 and 4 are larger than the thickness of the first mirror 30. Therefractive index of the first waveguide 1, i.e., the refractive index ofthe optical waveguide layer 2 of the first waveguide 1, is unchanged oris changed by an amount different from the amount of change in therefractive index of the optical waveguide layer 20. The thickness of theoptical waveguide layer 2 is unchanged or is changed by an amountdifferent from the amount of change in the thickness of the opticalwaveguide layer 20. The first waveguide 1 is connected directly to theoptical waveguide layer 20 of the second waveguide 10. For example, anend surface of the optical waveguide layer 2 of the first waveguide 1 isconnected to an end surface of the optical waveguide layer 20 of thesecond waveguide 10. The multilayer reflective film 3 in this examplehas a portion 3 a having a smaller thickness (i.e., lower reflectance)than its adjacent portion. Light is inputted from the portion 3 a(referred to also as a “light inputting portion 3 a”). By inputting thelight from the low-reflectance region, the light can be efficientlyintroduced into the optical waveguide layer 2. The optical waveguidelayer 2 propagates the light entering the light inputting portion 3 a,and the light is inputted to the end surface of the optical waveguidelayer 20 of the second waveguide 10. In this manner, the lightpropagates from the optical waveguide layer 2 to the optical waveguidelayer 20 and can be emitted through the mirror 30.

In the second waveguide 10, the reflectance of the multilayer reflectivefilm of the first mirror 30 is lower than the reflectance of themultilayer reflective film of the second mirror 40 because it isnecessary to emit light through the first mirror 30. The first waveguide1 is designed such that the reflectance of the multilayer reflectivefilms 3 and 4 is comparable to the reflectance of the second mirror 40in order to prevent light emission.

With the above-described structure, the optical scanning device canefficiently emit light from the second waveguide 10, as described later.

FIG. 13 is a cross-sectional view schematically showing another exampleof the structure of the optical scanning device. In this example, thefirst waveguide 1 includes no multilayer reflective films 3 and 4. Thefirst waveguide 1 propagates light by total reflection. The firstwaveguide 1 has a grating 5 on part of its surface. Light is inputtedthrough the grating 5. In this example, the portion in which the grating5 is disposed serves as a light inputting portion. By providing thegrating 5, the light can be easily introduced into the first waveguide1. When no multilayer reflective films 3 and 4 are provided as in thisexample, the first waveguide 1 is designed such that the angle θ_(w1) ofthe guided light satisfies the total reflection condition. Also in thiscase, the refractive index of the first waveguide 1 is unchanged or ischanged by an amount different from the amount of change in therefractive index of the optical waveguide layer 20. The thickness of thefirst waveguide 1, i.e., the thickness of the optical waveguide layer 2,is unchanged or is changed by an amount different from the amount ofchange in the thickness of the optical waveguide layer 20. The firstwaveguide 1 is connected directly to the optical waveguide layer 20 ofthe second waveguide 10. The optical waveguide layer 20 propagates thelight in the same direction as the guiding direction of the firstwaveguide 1.

FIG. 14 is a cross-sectional view schematically showing yet anotherexample of the structure of the optical scanning device. The opticalscanning device in this example further includes a third waveguide 1′connected to the first waveguide 1. The first waveguide 1 is areflective waveguide and includes two opposed multilayer reflectivefilms 3 and 4 and an optical waveguide layer 2 disposed therebetween.The third waveguide 1′ is a total reflection waveguide that propagateslight by total reflection. The refractive index of the third waveguide1′ is unchanged or is changed by an amount different from the amount ofchange in the refractive index of the optical waveguide layer 20. Thethickness of the third waveguide 1′, i.e., the thickness of an opticalwaveguide layer 2′, is unchanged or is changed by an amount differentfrom the amount of change in the thickness of the optical waveguidelayer 20. The third waveguide 1′ is connected directly to the opticalwaveguide layer 2 of the first waveguide 1. The optical waveguide layer20 propagates light in the same direction as the guiding direction ofthe third waveguide 1′. The third waveguide 1′ has a grating 5′ on partof its surface, as does the first waveguide 1 in the example in FIG. 13.Light from a light source is inputted to the third waveguide 1′ throughthe grating 5′. In this example, the portion in which the grating 5′ isdisposed serves as a light inputting portion. The refractive index orthickness of the optical waveguide layer 20 of the second waveguide 10is modulated by an unillustrated adjusting element (modulating element).No modulating function is provided for the first waveguide 1. To preventlight emission from the first waveguide 1, the reflectance of thereflecting mirrors (the multilayer reflective films 3 and 4) of thefirst waveguide 1 is set to be higher than the reflectance of the firstmirror 30 of the second waveguide 10. The reflectance of the firstmirror 30 of the second waveguide 10 is set to be lower than thereflectance of the second mirror 40. With this structure, the lightinputted into the third waveguide 1′ propagates through the thirdwaveguide 1′ and the first waveguide 1 and is inputted into the secondwaveguide 10. The inputted light is emitted to the outside through thefirst mirror 30 while propagating through the optical waveguide layer 20of the second waveguide 10.

FIGS. 15 and 16A to 16C are illustrations showing examples of a methodfor inputting light into the first waveguide 1 in a structure configuredsuch that the light is inputted to the first waveguide 1. FIG. 15 showsan example in which light enters an optical waveguide layer 2 sandwichedbetween two multilayer reflective films, as in the example shown in FIG.12. As shown in FIG. 15, by causing the light to be incident on asmall-thickness portion (i.e., low-reflectance portion) 3 a of amultilayer reflective film, the light can be efficiently introduced intothe optical waveguide layer 2. FIG. 16A shows an example in which lightis introduced into a first waveguide 1 through a grating 5 formed on asurface of the first waveguide 1, as in the example shown in FIG. 13.FIG. 16B shows an example in which light is inputted from an end surfaceof a first waveguide 1. FIG. 16C shows an example in which light isinputted from a laser light source 6 disposed on a surface of a firstwaveguide 1 through this surface. The structure shown in FIG. 16C isdisclosed in, for example, M. Lamponi et al., “Low-ThresholdHeterogeneously Integrated InP/SOI Lasers With a Double Adiabatic TaperCoupler,” IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 1, Jan. 1,2012, pp 76-78. The entire disclosure of this document is incorporatedherein. With any of the above structures, light can be efficientlyintroduced into the waveguide 1.

The light inputting methods shown in FIGS. 15 to 16C are applicable alsoto the structure using the third waveguide 1′ shown in FIG. 14. In theexample shown in FIG. 14, the grating 5′ is provided on part of asurface of the third waveguide 1′, but the grating 5′ may not beprovided. For example, the light inputting method shown in FIG. 16B or16C may be applied to the third waveguide 1′. When the light inputtingmethod shown in FIG. 16B is applied to the third waveguide 1′, the thirdwaveguide 1′ propagates the light entering from an end surface of thethird waveguide 1′, and the propagating light is inputted to an endsurface of the first waveguide 1. When the light inputting method shownin FIG. 16C is applied to the third waveguide 1′, light is inputted fromthe laser light source disposed on a surface of the third waveguide 1′through this surface. The third waveguide 1′ propagates the inputtedlight, and this light is inputted to the end surface of the firstwaveguide 1. The third waveguide 1′ is not necessarily a totalreflection waveguide and may be the reflective waveguide shown in FIG.15.

As shown in FIGS. 12 and 13, the refractive index of the opticalwaveguide layer 2 of the first waveguide 1 is denoted by n_(w1), and therefractive index of the optical waveguide layer 20 of the secondwaveguide 10 is denoted by n_(w2). The emergent angle of light from thesecond waveguide 10 is denoted by θ. The reflection angle of the guidedlight in the first waveguide 1 is denoted by θ_(w1), and the reflectionangle of the guided light in the second waveguide 10 is denoted byθ_(w2). As shown in FIG. 14, the refractive index of the opticalwaveguide layer 2′ of the third waveguide 1′ is denoted by n_(w3), andthe reflection angle of the guided light in the third waveguide 1′ isdenoted by θ_(w3). In the present embodiment, to allow light to beextracted from the second waveguide 10 to the outside (an air layerhaving a refractive index of 1), n_(w2) sin θ_(w2)=sin θ<1 holds.

<Principle of Coupling of Guided Light>

Referring next to FIGS. 12 and 13, the principle of coupling of theguided light between waveguides 1 and 10 will be described. For the sakeof simplicity, the light propagating through the waveguides 1 and 10 isapproximately assumed to be a ray of light. It is assumed that lightundergoes total reflection at the interfaces between the opticalwaveguide layer 20 and the upper and lower multilayer reflective filmsof the waveguide 10 and at the interfaces between the optical waveguidelayer 2 and the upper and lower multilayer reflective films of thewaveguide 1 (or the interfaces between the optical waveguide layer 2 andthe external medium). The thickness of the optical waveguide layer 2 ofthe first waveguide 1 is denoted by d₁, and the thickness of the opticalwaveguide layer 20 of the second waveguide 10 is denoted by d₂. Then,conditions that allow propagating light to be present in the waveguides1 and 10 are represented by the following formulas (5) and (6),respectively.

2d ₁ n _(w1) cos θ_(w1) =mλ  (5)

2d ₂ n _(w2) cos θ_(w2) =mλ  (6)

Here, λ is the wavelength of the light, and m is an integer of 1 ormore.

In consideration of the Snell's law at the interface between thewaveguides 1 and 10, formula (7) holds.

n _(w1) sin(90°−θ_(w1))=n _(w2) sin(90°−θ_(w2))  (7)

By modifying formula (7), formula (8) below is obtained.

n _(w1) cos θ_(w1) =n _(w2) cos θ_(w2)  (8)

Suppose that formulas (5) and (8) hold. Then formula (6) holds even whenn_(w2) changes, provided that d₁ is equal to d₂. Specifically, even whenthe refractive index of the optical waveguide layer 20 is changed, lightcan propagate from the optical waveguide layer 2 to the opticalwaveguide layer 20 efficiently.

To derive the above formulas, the light is assumed to be a ray of lightfor simplicity. In practice, since the thicknesses d₁ and d₂ arecomparative to the wavelength λ (at most 10 times the wavelength), theguided light has wave characteristics. Therefore, strictly speaking, itis necessary that the effective refractive indexes of the opticalwaveguide layers 2 and 20, instead of the refractive indexes of theirmaterials, must be used as the above refractive indexes n_(w1) andn_(w2). Even when the thickness d₁ of the optical waveguide layer 2 isnot the same as the thickness d₂ of the optical waveguide layer 20 or,strictly speaking, when formula (8) does not hold, light can be guidedfrom the optical waveguide layer 2 to the optical waveguide layer 20.This is because the light is transmitted from the optical waveguidelayer 2 to the optical waveguide layer 20 in a near field. Specifically,when the electric field distribution in the optical waveguide layer 2overlaps the electric field distribution in the optical waveguide layer20, light is transmitted from the optical waveguide layer 2 to theoptical waveguide layer 20.

The above discussion holds also for the guided light between the thirdwaveguide 1′ and the first waveguide 1 in the example shown in FIG. 14.

<Results of Computations>

To examine the effects of the present embodiment, the present inventorscomputed the coupling efficiency of light under various conditions.FIMMWAVE available from Photon Design was used for the computations.

First, the coupling efficiency in a structure in which both thewaveguides 1 and 10 were sandwiched between multilayer reflective filmsas shown in FIG. 12 was computed. In the following computations, themode order of light propagating from the waveguide 1 to the waveguide 10is m=2. When the mode order of light in the waveguide 1 is the same asthe mode order of light in the waveguide 10, the light is coupled by thesame principle. Therefore, the mode order of the light is not limited tom=2.

FIG. 17 shows the d₂ dependence of the coupling efficiency of guidedlight from the waveguide 1 to the waveguide 10 when n_(w1) is 1.45, d₁is 1.27 μm, and the wavelength λ is 1.55 μm. The horizontal axisrepresents a value obtained by dividing d₂ by a cutoff thicknessd_(cutoff) (=mλ/(2n_(w2))) when the guided light is assumed to be a rayof light. The vertical axis represents the coupling efficiencynormalized by setting the value of a peak to 1. The computations wereperformed from a lower limit value at which a cutoff conditionindicating that no guided light is allowed to be present is satisfied toan upper limit value at which light is emitted to the outside. Thecomputations were performed when n_(w2) was 1.3, 1.6, 1.9, 2.2, and 2.5.The center of the first waveguide 1 in its thickness direction matchesthe center of the second waveguide 10 in its thickness direction. As canbe seen from the results in FIG. 17, the larger d₂/d_(cutoff), thehigher the coupling efficiency. As d₂/d_(cutoff) decreases, the mode isnot allowed to be present, and the coupling efficiency decreases.

FIG. 18 shows the results of computations performed using the samemethod except that n_(w1) was changed to 3.48 and d₁ was changed to 0.5μm. Also in this case, the mode order of the light propagating from thewaveguide 1 to the waveguide 10 was m=2. However, as described above,the mode order of the light is not limited to m=2. As can be seen fromFIG. 18, the larger d₂/d_(cutoff), the higher the coupling efficiency.As d₂/d_(cutoff) decreases, the mode is not allowed to be present, andthe coupling efficiency decreases.

The reason that the mode is present (i.e., the guided light is coupled)even when d₂/d_(cutoff) is smaller than 1 in FIGS. 17 and 18 is that theeffective thickness of the optical waveguide layer 2 is larger than d₂because of penetration of the light when it is reflected from themultilayer reflective films. The upper limit of d₂ is a value at whichlight is no longer emitted to the outside. This value is determined byassuming that the guided light is a ray of light and undergoes totalreflection at the interfaces between each waveguide and the upper andlower multilayer reflective films thereof. Specifically, the upper limitis the value of d₂ when the reflection angle of the guided light isequal to the total reflection angle with respect to the air. In thiscase, the following formula (9) holds.

n _(w2) sin θ_(w2)=1  (9)

From formulas (6) and (9) and d_(cutoff)=mλ/(2n_(w2)), the followingformula (10) holds.

d ₂ /d _(cutoff) =n _(w2)/√(n _(w2) ²−1)  (10)

Because of the penetration of the guided light when it is reflected fromthe multilayer reflective films, the effective refractive index for theguided light becomes lower than n_(w2). Therefore, the upper limit of d₂is larger than that in formula (6).

Desirably, the coupling efficiency in the structure in the presentembodiment is higher than that in the structure shown in FIG. 6B. Forexample, on the condition that the coupling efficiency is 7% or more,which is higher than the peak value shown in FIG. 7, the followingrelations

0.95×d _(cutoff) <d ₂<1.5×d _(cutoff)

(0.95×mλ/(2n _(w2))<d ₂<1.5×mλ/(2n _(w2)))

are obtained from the results in FIGS. 17 and 18.

FIG. 19 is a graph showing the above results classified by whether thecoupling efficiency is 0.5 or more or less than 0.5, with the horizontalaxis representing d₂/d_(cutoff) and the vertical axis representing therefractive index ratio (|n_(w1)−n_(w2)|/n_(w1)). On the condition thatthe coupling efficiency is 0.5 (i.e., 50%) or more, it is necessary thatthe refractive index ratio be less than 0.4 and that0.95×d_(cutoff)<d₂<1.5×d_(cutoff).

In the present embodiment, the refractive index n_(w1) of the firstwaveguide 1 is larger than the refractive index n_(w2) of the secondwaveguide 10 (n_(w1)>n_(w2)). However, the present disclosure is notlimited to this structure, and n_(w1) n_(w2) may hold.

FIG. 20 is an illustration showing a structure in which the center, withrespect to the direction of thickness, of the optical waveguide layer 2of the first waveguide 1 is offset by Δz from the center, with respectto the direction of thickness, of the optical waveguide layer 20 of thesecond waveguide 10. When the center line, with respect to the thicknessdirection, of the optical waveguide layer 20 of the second waveguide 10is located on the light emitting side (the first mirror 30 side) of thecenter line, with respect to the thickness direction, of the opticalwaveguide layer 2 of the first waveguide 1 as shown in FIG. 20, the signof Δz is positive. Let Δd be the difference (the absolute difference)between the thickness d₁ of the optical waveguide layer 2 of the firstwaveguide 1 and the thickness d₂ of the optical waveguide layer 20 ofthe second waveguide 10. When Δz=Δd/2, the Z direction position of alower portion (the side opposite to the light emitting side) of theoptical waveguide layer 2 of the waveguide 1 matches the Z directionposition of a lower portion of the optical waveguide layer 20 of thewaveguide 10.

FIG. 21 is a graph showing the Δz dependence of the coupling efficiencyof light from the first waveguide 1 to the second waveguide 10. Theresults in FIG. 21 were obtained by computing the coupling efficiency bysetting n_(w1) to 2.2, the wavelength λ to 1.55 μm, n_(w2) to 2.2, andΔd to 0.12 μm at different values of Δz. The coupling efficiencynormalized by a value at Δz=0 is shown in FIG. 21. When the center linesof the optical waveguide layers 2 and 20 with respect to their thicknessdirection are offset in the Z direction, the coupling efficiency islower than that when Δz is zero (0). However, even when −Δd/2<Δz<Δd/2,the coupling efficiency is 90% or more of that at Δz=0 and can bemaintained at a relatively high level.

In the example shown in FIG. 13, the first waveguide 1 guides light bytotal reflection. Also in this structure, the same basic principle canbe used, and the guided light beams propagating through the waveguides 1and 10 can be coupled to each other. The d₂ dependence of the couplingefficiency of the guided light from the first waveguide 1 to the secondwaveguide 10 in the structure shown in FIG. 13 was also determined bycomputations. FIG. 22A shows the d₂ dependence of the couplingefficiency when n_(w1) is 2.2, d₁ is 0.7 μm, and the wavelength λ is1.55 μm. FIG. 22B shows the d₂ dependence of the coupling efficiencywhen n_(w1) is 3.48, d₁ is 0.46 μm, and the wavelength λ is 1.55 μm. Onthe condition that the coupling efficiency is 7% or more as in theabove-described example, the following relations

0.95×d _(cutoff) <d2<1.5×d _(cutoff)

(i.e., 0.95×mλ/(2n _(w2))<d2<1.5×mλ/(2n _(w2)))

are obtained.

On the condition that the coupling efficiency is 50% or more, thefollowing conditions 1.2×d_(cutoff)<d₂<1.5×d_(cutoff) (i.e.,1.2×mλ/(2n_(w2))<d₂<1.5×mλ/(2n_(w2))) is obtained.

Also in the structure in FIG. 13, n_(w1)>n_(w2) may hold, orn_(w1)≤n_(w2) may hold.

As described above, the mode order of light propagating from thewaveguide 1 to the waveguide 10 is not limited to m=2. For example, whena model shown in FIG. 23A was used for the computations under theconditions of n_(w1)=1.883, d₁=0.3 μm, n_(w2)=1.6, and d₂=0.55 μm, lightwas coupled into the waveguide as shown in FIG. 23B.

Next, a structure in which a gap is present between the first waveguide1 and the second waveguide 10 will be studied.

FIG. 24A is a cross-sectional view showing a modification of the presentembodiment. In this example, the optical waveguide layer 20 of thesecond waveguide 10 is connected to the first waveguide 1 through a gap(e.g., an air gap). Even when the gap is present between the firstwaveguide 1 and the second waveguide 10 as described above, the light iscoupled in the near field of the waveguide mode. Therefore, when thewidth of the gap (the width in the X direction) is sufficiently smallerthan the wavelength λ, the guided light is coupled between thewaveguides 1 and 10. This differs from the coupling of the lightpropagating in free space to the waveguide mode in FIG. 6A or 6B.

FIG. 24B is a graph showing the results of computations of the gap widthdependence of the coupling efficiency. The coupling efficiencynormalized by a value when the gap is 0 μm is shown in FIG. 24B. In thecomputations, n_(w1) is 3.48, n_(w2) is 1.5. d₁ is 0.9 μm, and d₂ is 1.1μm. The refractive index of the gap is 1, and the wavelength λ is 1.55μm. As can be seen from FIG. 24B, the normalized coupling efficiency is50% or more when the gap is 0.24 μm or less. In consideration of thecase where the gap is a medium other than air and the case where thewavelength λ differs from 1.55 μm, when the optical length of the gap(the product of the refractive index of the gap and the gap width) isequal to or less than λ/6.5, the normalized coupling efficiency is 50%or more. The optical length of the gap does not depend on the parametersof the waveguides 1 and 10.

Also when light is inputted to the first waveguide 1 from the thirdwaveguide 1′ as in the example shown in FIG. 14, a gap may be presentbetween an end surface of the third waveguide 1′ and an end surface ofthe first waveguide 1. As described above, the optical length of the gap(the product of the refractive index of the gap and the gap width) isset to be, for example, λ/6.5 or less.

Next, a description will be given of a structure for implementingtwo-dimensional optical scanning using a plurality of pairs of the firstand second waveguides 1 and 10 in the present embodiment (these arereferred to as “waveguide units” in the present specification). Anoptical scanning device that can implement two-dimensional scanningincludes: a plurality of waveguide units arranged in a first direction;and an adjusting element (e.g., a combination of an actuator and acontrol circuit) that controls the waveguide units. The adjustingelement changes at least one of the refractive index and thickness ofthe optical waveguide layer 20 of the second waveguide 10 of each of thewaveguide units. In this manner, the direction of light emitted from thesecond waveguides 10 can be changed. When light beams with appropriatelycontrolled phase differences are inputted to the second waveguides 10 ofthe plurality of waveguide units, two-dimensional optical scanning canbe performed as described with reference to FIG. 1. An embodiment forimplementing two-dimensional scanning will next be described in moredetail.

<Operating Principle of Two-Dimensional Scanning>

In a waveguide array in which a plurality of waveguide elements (secondwaveguides) 10 are arranged in one direction, interference of lightbeams emitted from the waveguide elements 10 causes the emissiondirection of the light to change. By controlling the phases of the lightbeams to be supplied to the waveguide elements 10, the emissiondirection of the light can be changed. The principle of this will nextbe described.

FIG. 25A is an illustration showing a cross section of the waveguidearray that emits light in a direction perpendicular to the emissionsurface of the waveguide array. In FIG. 25A, phase shift amounts of thelight beams propagating through the waveguide elements 10 are shown. Thephase shift amounts are values with respect to the phase of a light beampropagating through the leftmost waveguide element 10. The waveguidearray in the present embodiment includes the plurality of waveguideelements 10 arranged at regular intervals. In FIG. 25A, broken line arcsrepresent wave fronts of the light beams emitted from the waveguideelements 10. A straight line represents a wave front formed as a resultof interference of the light beams. An arrow represents the direction ofthe light emitted from the waveguide array (i.e., the direction of thewave vector). In the example in FIG. 25A, the phases of the light beamspropagating through the optical waveguide layers 20 of the waveguideelements 10 are the same. In this case, light is emitted in a direction(the Z direction) perpendicular to the arrangement direction (the Ydirection) of the waveguide elements 10 and to the extending direction(the X direction) of the optical waveguide layers 20.

FIG. 25B is an illustration showing a cross section of the waveguidearray that emits light in a direction different from the directionperpendicular to the emission surface of the waveguide array. In theexample in FIG. 25B, the phases of the light beams propagating throughthe optical waveguide layers 20 of the plurality of waveguide elements10 differ from each other in the arrangement direction by a constantamount (Δϕ). In this case, light is emitted in a direction differentfrom the Z direction. By changing Δϕ, the Y direction component of thewave vector of the light can be changed.

The direction of the light emitted from the waveguide array to theoutside (air in this case) can be quantitatively discussed as follows.

FIG. 26 is a perspective view schematically showing the waveguide arrayin a three-dimensional space. In the three-dimensional space defined bymutually orthogonal X, Y, and Z directions, a boundary surface betweenthe waveguide array and a region to which light is emitted to air is setto be Z=z₀. The boundary surface contains the emission surfaces of theplurality of waveguide elements 10. In a region in which Z<z₀ holds, theplurality of waveguide elements 10 are arranged in the Y direction atregular intervals and extend in the X direction. In a region in whichZ>z₀ holds, the electric-field vector E(x, y, z) of light emitted to airis represented by formula (11) below.

E(x,y,z)=E ₀ exp[−j(k _(x) x+k _(y) y+k _(z) z)]  (11)

Here, E₀ is the amplitude vector of the electric field. k_(x), k_(y),and k_(z) are the wave numbers in the X, Y, and Z directions,respectively, and j is the imaginary unit. In this case, the directionof the light emitted to air is parallel to a wave vector (k_(x), k_(y),k_(z)) indicated by a thick arrow in FIG. 26. The magnitude of the wavevector is represented by formula (12) below.

$\begin{matrix}{\sqrt{k_{x}^{2} + k_{y}^{2} + k_{z}^{2}} = \frac{2\pi}{\lambda}} & (12)\end{matrix}$

Wave vector components k_(x) and k_(y) parallel to the boundary surfaceagree with the wave numbers of light in the X and Y directions,respectively, in the waveguide array. This corresponds to the conditionin which the wavelengths, in the plane directions, of the light on theair side at the boundary surface agree with the wavelengths, in theplane directions, of the light on the waveguide array side, as in theSnell's law in formula (2).

k_(x) is equal to the wave number of the light propagating through theoptical waveguide layer 20 of a waveguide element 10 extending in the Xdirection. In the waveguide element 10 shown in FIG. 2 above, k_(x) isrepresented by formula (13) below using formulas (2) and (3).

$\begin{matrix}{k_{x} = {{\frac{2\pi}{\lambda}n_{w}\sin \; \theta_{w}} = {\frac{2\pi}{\lambda}\sqrt{n_{w}^{2} - \left( \frac{m\; \lambda}{2d} \right)^{2}}}}} & (13)\end{matrix}$

k_(y) is derived from the phase difference between light beams in twoadjacent waveguide elements 10. The centers of N waveguide elements 10arranged in the Y direction at regular intervals are denoted by y_(q)(q=0, 1, 2, . . . , N−1), and the distance (center-to-center distance)between two adjacent waveguide elements 10 is denoted by p. In thiscase, the electric-field vectors (formula (11)) of light emitted to airat y_(q) and y_(q+1) on the boundary surface (Z=z₀) satisfy thefollowing formula.

E(x,y _(q+1) ,z ₀)=exp[(−jk _(y)(y _(q+1) −y _(q))]E(x,y _(q) ,z₀)=exp[−jk _(y) p]E(x,y _(q) ,z ₀)  (14)

When the phases in any two adjacent waveguide elements are set such thatthe phase difference is Δϕ=k_(y)p (constant), k_(y) satisfies therelation of formula (15) below.

$\begin{matrix}{k_{y} = \frac{\Delta \; \varphi}{p}} & (15)\end{matrix}$

In this case, the phase of light at y_(q) is represented byϕ_(q)=ϕ₀+qΔϕ(ϕ_(q+1)−ϕ_(q)=Δϕ). Specifically, the phase ϕ_(q) isconstant (Δϕ=0), linearly increases in the Y direction (Δϕ>0), orlinearly decreases in the Y direction (Δϕ<0). When the waveguideelements 10 are arranged in the Y direction at non-regular intervals,the phases at y_(q) and y_(q+1) are set such that the phase differencefor a given k_(y) is Δϕ_(q)=ϕ_(q+1)−ϕ_(q)=k_(y)(y_(q+1)−y_(q)). In thiscase, the phase of the light at y_(q) is represented byϕ_(q)=ϕ₀+k_(y)(y_(q)−y₀). Using k_(x) and k_(y) obtained from formulas(14) and (15), respectively, k_(z) is derived from formula (12). Theemission direction of the light (the direction of the wave vector) canthereby be obtained.

For example, as shown in FIG. 26, the angle between the wave vector(k_(y), k_(y), k_(z)) of the emitted light and a vector (0, k_(y),k_(z)) obtained by projecting the wave vector onto the YZ plane isdenoted by θ. θ is the angle between the wave vector and the YZ plane. θis represented by formula (16) below using formulas (12) and (13).

$\begin{matrix}{{\sin \; \theta} = {\frac{k_{x}}{\sqrt{k_{x}^{2} + k_{y}^{2} + k_{z}^{2}}} = {{\frac{\lambda \;}{2\pi}k_{x}} = \sqrt{n_{w}^{2} - \left( \frac{m\; \lambda}{2d} \right)^{2}}}}} & (16)\end{matrix}$

Formula (16) is exactly the same as formula (3) derived when the emittedlight is restricted to be parallel to the XZ plane. As can be seen fromformula (16), the X component of the wave vector changes depending onthe wavelength of the light, the refractive index of the opticalwaveguide layers 20, and the thickness of the optical waveguide layers20.

Similarly, as shown in FIG. 26, the angle between the wave vector(k_(y), k_(y), k_(z)) of the emitted light (zeroth-order light) and avector (k_(y), 0, k_(z)) obtained by projecting the wave vector onto theXZ plane is denoted by α₀. α₀ is the angle between the wave vector andthe XZ plane. α₀ is represented by formula (17) below using formulas(12) and (13).

$\begin{matrix}{{\sin \; \alpha_{0}} = {\frac{k_{y}}{\sqrt{k_{x}^{2} + k_{y}^{2} + k_{z}^{2}}} = {{\frac{\lambda \;}{2\pi}k_{y}} = \frac{\Delta \; \varphi \; \lambda}{2\pi \; p}}}} & (17)\end{matrix}$

As can be seen from formula (17), the Y component of the wave vector ofthe light changes depending on the phase difference Δϕ of the light.

As described above, θ and α₀ obtained from formulas (16) and (17),respectively, may be used instead of the wave vector (k_(y), k_(y),k_(z)) to identify the emission direction of the light. In this case,the unit vector representing the emission direction of the light can berepresented by (sin θ, sin α₀, (1−sin²α₀−sin² θ)^(1/2)). For lightemission, all these vector components must be real numbers, andtherefore sin² α₀+sin² θ≤1 is satisfied. Since sin² α₀≤1−sin² θ=cos² θ,the emitted light is changed within an angle range in which −cos θ≤sinα₀≤cos θ is satisfied. Since −1≤sin α₀≤1, the emitted light is changedwithin the angle range of −90°≤α₀≤90° at α₀=0°. However, as θ increases,cos θ decreases, so that the angle range of α₀ is narrowed. When θ=90°(cos θ=0), light is emitted only at α₀=0°.

The two-dimensional optical scanning in the present embodiment can beimplemented using at least two waveguide elements 10. When the number ofwaveguide elements 10 is small, the divergence angle Δα of α₀ is large.As the number of waveguide elements 10 increases, Δα decreases. This canbe explained as follows. For the sake of simplicity, θ is assumed to be0° in FIG. 26. Specifically, the emission direction of the light isparallel to the YZ plane.

Assume that light beams having the same emission intensity and theabove-described phases ϕ_(q) are emitted from N waveguide elements 10 (Nis an integer of 2 or more). In this case, the absolute value of thetotal amplitude distribution of the light beams (electric fields)emitted from the N waveguide elements 10 in a far field is proportionalto F(u) represented by formula (18) below.

$\begin{matrix}{{F(u)} = {\frac{\sin \left( {N\; {u/2}} \right)}{\sin \left( {u/2} \right)}}} & (18)\end{matrix}$

Here, u is represented by formula (19) below.

$\begin{matrix}{u = {\frac{2\pi \; p}{\lambda}\left( {{\sin \; \alpha} - {\sin \; \alpha_{0}}} \right)}} & (19)\end{matrix}$

Here, α is the angle between the Z axis and a line connecting the originand an observation point in the YZ plane. α₀ satisfies formula (17).F(u) in formula (18) is N (maximum) when u=0 (α=α₀) and is 0 whenu=±2π/N. Let the angle satisfying u=−2π/N be α₁, and the anglesatisfying u=2π/N be α₂ (α₁<α₀<α₂). Then the divergence angle of α₀ isΔα=α₂−α₁. A peak within the range of −2π/N<u<2π/N (α₁<α<α₂) is generallyreferred to as a main lobe. A plurality of small peaks referred to asside lobes are present on both sides of the main lobe. By comparing thewidth Δu=4π/N of the main lobe and Δu=2πpΔ(sin α)/λ obtained fromformula (19), Δ(sin α)=2λ/(Np) is obtained. When Δα is small, Δ(sinα)=sin α₂−sin α₁=[(sin α₂−sin α₁)/(α₂−α₁)]Δα=[d(sin α)/dα]_(α=α0) Δα=cosα₀ Δα. Therefore, the divergence angle is represented by formula (20)below.

$\begin{matrix}{{\Delta \; \alpha} = \frac{2\lambda}{N\; p\; \cos \; \alpha_{0}}} & (20)\end{matrix}$

Thus, as the number of waveguide elements 10 increases, the divergenceangle Δα decreases, and high resolution optical scanning can beperformed on a distant target. The same discussion is applicable to thecase when θ≠0° in FIG. 26.<Diffracted Light Emitted from Waveguide Array>

In addition to the zeroth-order light beam, higher-order diffractedlight beams may be emitted from the waveguide array. For the sake ofsimplicity, θ is assumed to be 0° in FIG. 26. Specifically, the emissiondirection of the diffracted light is parallel to the YZ plane.

FIG. 27A is a schematic diagram showing how diffracted light is emittedfrom the waveguide array when p is larger than λ. In this case, whenthere is no phase shift (α₀=0°), zeroth-order and ±first-order lightbeams are emitted in directions indicated by solid arrows shown in FIG.27A (higher-order diffracted light beams may be emitted, but thisdepends on the magnitude of p). When a phase shift is given to thisstate (α₀≠0°), the emission angles of the zeroth-order and ±first-orderlight beams rotate in the same rotation direction as shown by brokenline arrows in FIG. 27A. Higher-order light beams such as the±first-order light beams can be used for beam scanning. However, toconfigure a simpler device, only the zeroth-order light beam is used. Toavoid a reduction in gain of the zeroth-order light beam, the distance pbetween two adjacent waveguide elements 10 may be reduced to be lessthan λ to suppress the emission of higher-order light beams. Even whenp>λ, only the zeroth-order light beam can be used by physically blockingthe higher-order light beams.

FIG. 27B is a schematic diagram showing how diffracted light is emittedfrom the waveguide array when p is smaller than λ. In this case, whenthere is no phase shift (α₀=0°), no higher-order light beams are presentbecause the diffraction angles of the higher-order light beams exceed 90degrees, and only the zeroth-order light beam is emitted forward.However, in the case where p is close to λ, when a phase shift is given(α₀≠0°), the emission angles change, and the ±first-order light beamsmay be emitted. FIG. 27C is a schematic diagram showing how diffractedlight is emitted from the waveguide array when p≈λ/2. In this case, evenwhen a phase shift is given (α₀≠0°), the ±first-order light beams arenot emitted. Even when the ±first-order light beams are emitted, theyare emitted at considerably large angles. When p<λ/2, even when a phaseshift is given, no higher-order light beams are emitted. However, evenwhen p is further reduced, no particular advantage is expected.Therefore, p may be set to be, for example, λ/2 or more.

The relation between the zeroth-order light beam and ±first-order lightbeams emitted to air in FIGS. 27A to 27C can be quantitively explainedas follows. F(u) in formula (18) is F(u)=F(u+2π) and is a function witha period of 2π. When u=±2mπ, F(u)=N (maximum). In this case, ±m-th orderlight beams are emitted at emission angles α satisfying u=±2mπ. Peaksaround u=±2mπ(m≠0) (peak width: Δu=4π/N) are referred to as gratinglobes.

Only ±first-order light beams contained in higher-order light areconsidered (u=±2π). The emission angles α± of the ±first-order lightbeams satisfy formula (21) below.

$\begin{matrix}{{\sin \; \alpha_{\pm}} = {{\sin \; \alpha_{0}} \pm \frac{\lambda}{p}}} & (21)\end{matrix}$

p<λ/(1−sin α₀) is obtained from the condition sin α₀>1 indicating thatthe +first-order light beam is not emitted. Similarly, p<λ/(1+sin α₀) isobtained from the condition sin α₀<−1 indicating that the −first-orderlight beam is not emitted.

Conditions indicating whether or not the ±first-order light beams areemitted in addition to the zeroth-order light beam at an emission angleα₀ (>0) are classified as follows. When p≥λ/(1−sin α₀), both±first-order light beams are emitted. When λ/(1+sin α₀)≤p<λ/(1−sin α₀),the +first-order light beam is not emitted, but the −first-order lightbeam is emitted. When p<λ/(1+sin α₀), the ±first-order light beams arenot emitted. In particular, when p<λ/(1+sin α₀) is satisfied, the±first-order light beams are not emitted even when θ≠0° in FIG. 26. Forexample, to achieve scanning over 10° on one side when the ±first-orderlight beams are not emitted, α₀ is set to 10°, and it is only necessarythat the relation p λ/(1+sin 10°)≈0.85λ be satisfied. Using theabove-described lower limit of p in combination, it is only necessary tosatisfy λ/2≤p≤λ/(1+sin 10°).

However, to satisfy the condition that the ±first-order light beams arenot emitted, p must be very small. This makes it difficult to producethe waveguide array. Therefore, it is contemplated that the angle rangeof 0°<α₀<α_(max) is scanned with the zeroth-order light beamirrespective of the presence or absence of the ±first-order light beams.However, it is assumed that the ±first-order light beams are not presentin this angle range. To satisfy this condition, the emission angle ofthe +first-order light beam when α₀=0° must be α₊≥α_(max) (i.e., sinα₊=(λ/p)≥sin α_(max)), and the emission angle of the −first-order lightbeam when α₀=α_(max) must be α⁻≤0 (i.e., sin α⁻=sin α_(max)−(λ/p)≤0).These restrictions give p≤λ/sin α_(max).

As can be seen from the above discussion, the maximum value α_(max) ofthe emission angle α₀ of the zeroth-order light beam when the±first-order light beams are not present within the scanning angle rangesatisfies formula (22) below.

$\begin{matrix}{{\sin \; \alpha_{\max}} = \frac{\lambda}{p}} & (22)\end{matrix}$

For example, to achieve scanning over 10° on one side when the±first-order light beams are not present within the scanning anglerange, α₀ is set to 10°, and it is necessary that the relation p≤λ/sin10°≈5.76λ be satisfied. Using the above-described condition for thelower limit of p in combination, p satisfies λ/2≤p≤λ/sin 10°. Since thisupper limit of p (p≈5.76λ) is sufficiently larger than the upper limit(p≈0.85λ) when the ±first-order light beams are not emitted, thewaveguide array can be produced relatively easily. When the light usedis not single-wavelength light, λ is the center wavelength of the lightused.

As described above, to scan over a wider angle range, it is necessary toreduce the distance p between waveguides. However, to reduce thedivergence angle Δα of the emitted light in formula (20) when p issmall, it is necessary to increase the number of waveguides in thewaveguide array. The number of waveguides in the waveguide array isappropriately determined according to its intended application and therequired performance. The number of waveguides in the waveguide arraymay be, for example, 16 or more and may be 100 or more in someapplications.

<Phase Control of Light Introduced into Waveguide Array>

To control the phase of light emitted from each waveguide element 10, aphase shifter that changes the phase of the light before introductioninto the waveguide element 10 is installed upstream of the waveguideelement 10. The optical scanning device 100 in the present embodimentfurther includes a plurality of phase shifters connected to therespective waveguide elements 10 and a second adjusting element thatchanges the phases of light beams propagating through of the phaseshifters. Each phase shifter includes a waveguide that is connected tothe optical waveguide layer 20 of a corresponding one of the pluralityof waveguide elements 10 directly or through another waveguide. Thesecond adjusting element changes the phase differences between the lightbeams propagating from the plurality of phase shifters to the pluralityof waveguide elements 10 to thereby change the direction (the thirddirection D3) of light emitted from the plurality of waveguide elements10. In the following description, the plurality of arranged phaseshifters may be referred to as a “phase shifter array,” as in the caseof the waveguide array.

FIG. 28 is a schematic diagram showing an example of a structure inwhich a phase shifter 80 is connected directly to a waveguide element10. In FIG. 28, a portion surrounded by a broken line frame correspondsto the phase shifter 80. The phase shifter 80 includes a pair of opposedmirrors (a fifth mirror 30 a and a sixth mirror 40 a which may bereferred to simply as mirrors) and a waveguide 20 a disposed between themirrors 30 a and 40 a. The waveguide 20 a in this example is formed ofthe same material as the material of the optical waveguide layer 20 ofthe waveguide element 10 and is connected directly to the opticalwaveguide layer 20. Similarly, the mirror 40 a is formed of the samematerial as the material of the mirror 40 of the waveguide element 10and is connected to the mirror 40. The mirror 30 a has a lowertransmittance (higher reflectance) than the mirror 30 of the waveguideelement 10. The mirror 30 a is connected to the mirror 30. The phaseshifter 80 is designed such that the transmittance of the mirror 30 a isas low as that of the mirrors 40 and 40 a in order not to emit light.Specifically, the light transmittance of the fifth mirror 30 a and thelight transmittance of the sixth mirror 40 a are lower than the lighttransmittance of the first mirror 30. In this example, the phase shifter80 corresponds to the “first waveguide” in the present disclosure. The“first waveguide” may serve as the phase shifter as described above.

FIG. 29 is a schematic diagram of a waveguide array 10A and a phaseshifter array 80A as viewed in a direction normal to a light-emissionsurface (in the Z direction). In the example shown in FIG. 29, all thephase shifters 80 have the same propagation characteristics, and all thewaveguide elements 10 have the same propagation characteristics. Thephase shifters 80 may have the same length or may have differentlengths, and the waveguide elements 10 may have the same length or mayhave different lengths. When the phase shifters 80 have the same length,a driving voltage may be changed to control the phase shift amount ofeach of the phase shifters 80. When the phase shifters 80 have lengthsthat differ in equal steps, the same driving voltage can be used to givephase shifts that differ in equal steps. This optical scanning device100 further includes an optical divider 90 that divides light andsupplies divided light beams to the plurality of phase shifters 80, afirst driving circuit 110 that drives each of the waveguide elements 10,and a second driving circuit 210 that drives each of the phase shifters80. A straight arrow in FIG. 29 indicates light input. The first drivingcircuit 110 and the second driving circuit 210 that are disposedseparately are controlled independently to implement two-dimensionalscanning. In this example, the first driving circuit 110 serves as acomponent of the first adjusting element, and the second driving circuit210 serves as a component of the second adjusting element.

As described later, the first driving circuit 110 changes (modulates)the refractive index or thickness of the optical waveguide layer 20 ofeach of the waveguide elements 10 to thereby change the angle of lightemitted from the optical waveguide layer 20. As described later, thesecond driving circuit 210 changes the refractive index of the waveguide20 a of each of the phase shifters 80 to thereby change the phase oflight propagating inside the waveguide 20 a. The optical divider 90 maybe composed of waveguides in which light propagates by total reflectionor reflective waveguides similar to the waveguide elements 10.

The phases of light beams divided by the optical divider 90 may becontrolled, and then the resulting light beams may be introduced intothe phase shifters 80. To control the phases, for example, a passivephase control structure in which the lengths of waveguides connected tothe phase shifters 80 are adjusted to control the phases of the lightbeams may be used. Alternatively, phase shifters that have the samefunction as the phase shifters 80 and are controllable using an electricsignal may be used. By using any of these methods, the phases of thelight beams may be adjusted before they are introduced into the phaseshifters 80 such that, for example, light beams having the same phaseare supplied to all the phase shifters 80. By adjusting the phases asdescribed above, the second driving circuit 210 can control each of thephase shifters 80 in a simpler manner.

FIG. 30 is an illustration schematically showing an example of astructure in which the waveguides of the phase shifters 80 are connectedto the optical waveguide layers 20 of the waveguide elements 10 throughadditional waveguides 85. Each of the additional waveguides 85 may beany of the above-described first waveguides 1. Each additional waveguide85 may be a combination of the waveguides 1 and 1′ shown in FIG. 14.Each phase shifter 80 may have the same structure as the phase shifter80 shown in FIG. 28 or may have a different structure. In FIG. 30, thephase shifters 80 are simply represented by symbols ϕ₀ to ϕ₅ thatindicate the phase shift amounts. The same representation may be used inlater figures. A waveguide that can propagate light using totalreflection may be used for each phase shifter 80. In this case, themirrors 30 a and 40 a shown in FIG. 28 are not necessary.

FIG. 31 is an illustration showing a structural example in which aplurality of phase shifters 80 arranged in a cascaded manner areinserted into the optical divider 90. In this example, the plurality ofphase shifters 80 are connected to intermediate points of a channel ofthe optical divider 90. The phase shifters 80 give the same phase shiftamount ϕ to light propagating therethrough. When the phase shift amountsgiven by the phase shifters 80 are the same, the phase differencesbetween any two adjacent waveguide elements 10 are the same. Therefore,the second adjusting element can transmit a common phase control signalto all the phase shifters 80. This is advantageous in that the structureis simplified.

Waveguides can be used to efficiently propagate light between theoptical divider 90, the phase shifters 80, the waveguide elements 10,etc. An optical material having a higher refractive index than itssurrounding material and absorbing less light can be used for thewaveguides. For example, materials such as Si, GaAs, GaN, SiO₂, TiO₂,Ta₂O₅, AlN, and SiN can be used. Any of the above-described firstwaveguides 1 may be used to propagate light from the optical divider 90to the waveguide elements 10. To propagate light from the opticaldivider 90 to the waveguide elements 10, the waveguides 1 and 1′ shownin FIG. 14 may be used.

The phase shifters 80 require a mechanism for changing a light pathlength in order to give a phase difference to light. In the presentembodiment, the refractive index of the waveguide of each phase shifter80 is modulated to change the light path length. In this manner, thephase difference between light beams to be supplied from two adjacentphase shifters 80 to their respective waveguide elements 10 can beadjusted. More specifically, the refractive index of a phase shiftmaterial in the waveguide of each phase shifter 80 is modulated, and thephase shift can thereby be given. A specific example of the structurefor refractive index modulation will be described later.

<Examples of First Adjusting Element>

Next, a description will be given of structural examples of the firstadjusting element that adjusts the refractive index or thickness of theoptical waveguide layer 20 of each waveguide element 10. First, astructural example when the refractive index is adjusted will bedescribed.

FIG. 32A is a perspective view schematically showing an example of thestructure of the first adjusting element 60 (hereinafter may be referredto simply as an adjusting element). In the example shown in FIG. 32A,the adjusting element 60 includes a pair of electrodes 62 and isinstalled in the waveguide element 10. The optical waveguide layer 20 issandwiched between the pair of electrodes 62. The optical waveguidelayer 20 and the pair of electrodes 62 are disposed between a firstmirror 30 and a second mirror 40. The entire side surfaces (the surfacesparallel to the XZ plane) of the optical waveguide layer 20 are incontact with the electrodes 62. The optical waveguide layer 20 containsa refractive index modulatable material whose refractive index for thelight propagating through the optical waveguide layer 20 is changed whena voltage is applied. The adjusting element 60 further includes wiringlines 64 led from the pair of electrodes 62 and a power source 66connected to the wiring lines 64. By turning on the power source 66 toapply a voltage to the pair of electrodes 62 through the wiring lines64, the refractive index of the optical waveguide layer 20 can bemodified. Therefore, the adjusting element 60 may be referred to as arefractive index modulatable element.

FIG. 32B is a perspective view schematically showing another example ofthe structure of the first adjusting element 60. In this example, onlyparts of the side surfaces of the optical waveguide layer 20 are incontact with the electrodes 62. The rest of the structure is the same asthat shown in FIG. 32A. Even with the structure in which the refractiveindex of part of the optical waveguide layer 20 is changed, thedirection of emitted light can be changed.

FIG. 32C is a perspective view schematically showing yet another exampleof the structure of the first adjusting element 60. In this example, thepair of electrodes 62 have a layer shape approximately parallel to thereflecting surfaces of the mirrors 30 and 40. One of the electrodes 62is sandwiched between the first mirror 30 and the optical waveguidelayer 20. The other electrode 62 is sandwiched between the second mirror40 and the optical waveguide layer 20. When this structure is employed,transparent electrodes may be used as the electrodes 62. This structureis advantageous in that it can be produced relatively easily.

In the examples shown in FIGS. 32A to 32C, the optical waveguide layer20 of each waveguide element 10 contains a material whose refractiveindex for the light propagating through the optical waveguide layer 20is changed when a voltage is applied. The first adjusting element 60includes the pair of electrodes 62 sandwiching the optical waveguidelayer 20 and changes the refractive index of the optical waveguide layer20 by applying a voltage to the pair of electrodes 62. The voltage isapplied using the first driving circuit 110 described above.

Examples of the materials used for the above components will bedescribed.

The material used for the mirrors 30, 40, 30 a, and 40 a may be, forexample, a dielectric multilayer film. A mirror using a multilayer filmcan be produced by, for example, forming a plurality of films having anoptical thickness of ¼ wavelength and having different refractiveindexes periodically. Such a multilayer film mirror can have highreflectance. The materials of the films used may be, for example, SiO₂,TiO₂, Ta₂O₅, Si, and SiN. The mirrors are not limited to multilayer filmmirrors and may be formed of a metal such as Ag or Al.

Various conductive materials can be used for the electrodes 62 and thewiring lines 64. For example, conductive materials including metalmaterials such as Ag, Cu, Au, Al, Pt, Ta, W, Ti, Rh, Ru, Ni, Mo, Cr, andPd, inorganic compounds such as ITO, tin oxide, zinc oxide, IZO(registered trademark), and SRO, and conductive polymers such as PEDOTand polyaniline can be used.

Various light-transmitting materials such as dielectric materials,semiconductors, electrooptical materials, and liquid crystal moleculescan be used for the material of the optical waveguide layer 20. Examplesof the dielectric materials include SiO₂, TiO₂, Ta₂O₅, SiN, and AlN.Examples of the semiconductor materials include Si-based, GaAs-based,and GaN-based materials. Examples of the electrooptical materialsinclude lithium niobate (LiNbO₃), barium titanate (BaTiO₃), lithiumtantalate (LiTaO₃), zinc oxide (ZnO), lead lanthanum zirconate titanate(PLZT), and potassium tantalate niobate (KTN).

To modulate the refractive index of the optical waveguide layer 20, forexample, methods utilizing a carrier injection effect, an electroopticaleffect, a birefringent effect, and a thermooptical effect can be used.Examples of these methods will next be described.

The method utilizing the carrier injection effect can be implemented bya structure utilizing a pin junction of semiconductors. In this method,a structure in which a semiconductor with a low dopant concentration issandwiched between a p-type semiconductor and an n-type semiconductor isused, and the refractive index of the semiconductor is modulated byinjecting carriers into the semiconductor. In this structure, theoptical waveguide layer 20 of each of the waveguide elements 10 containsa semiconductor material. One of the pair of electrodes 62 may contain ap-type semiconductor, and the other one may contain an n-typesemiconductor. In the first adjusting element 60, a voltage is appliedto the pair of electrodes 62 to inject carriers into the semiconductormaterial, and the refractive index of the optical waveguide layer 20 isthereby changed. Specifically, the optical waveguide layer 20 may beproduced using a non-doped or low-dopant concentration semiconductor,and the p-type semiconductor and the n-type semiconductor may bedisposed in contact with the optical waveguide layer 20. A complexstructure may be used in which the p-type semiconductor and the n-typesemiconductor are disposed in contact with the low-dopant concentrationsemiconductor and conductive material layers are in contact with thep-type semiconductor and the n-type semiconductor. For example, whencarriers of about 10²⁰ cm⁻³ are injected into Si, the refractive indexof Si is changed by about 0.1 (see, for example, “Free charge carrierinduced refractive index modulation of crystalline Silicon,” 7^(th) IEEEInternational Conference on Group IV Photonics, P102-104, 1-3 Sep.2010). When this method is used, a p-type semiconductor and an n-typesemiconductor may be used as the materials of the pair of electrodes 62in FIGS. 32A to 32C. Alternatively, the pair of electrodes 62 may beformed of a metal, and the optical waveguide layer 20 itself or layersbetween the optical waveguide layer 20 and the electrodes 62 may containa p-type or n-type semiconductor.

The method utilizing the electrooptical effect can be implemented byapplying a voltage to an optical waveguide layer 20 containing anelectrooptical material. In particular, when KTN is used as theelectrooptical material, the electrooptical effect obtained can belarge. The relative dielectric constant of KTN increases significantlyat a temperature slightly higher than its tetragonal-to-cubic phasetransition temperature, and this effect can be utilized. For example,according to “Low-Driving-Voltage Electro-Optic Modulator With NovelKTa1-xNbxO3 Crystal Waveguides,” Jpn. J. Appl. Phys., Vol. 43, No. 8B(2004), an electrooptical constant of g=4.8×10⁻¹⁵ m²/V² is obtained forlight with a wavelength of 1.55 μm. For example, when an electric fieldof 2 kV/mm is applied, the refractive index is changed by about 0.1(=gn³E³/2). With the structure utilizing the electrooptical effect, theoptical waveguide layer 20 of each of the waveguide elements 10 containsan electrooptical material such as KTN. The first adjusting element 60changes the refractive index of the electrooptical material by applyinga voltage to the pair of electrodes 62.

In the method utilizing the birefringent effect of a liquid crystal, anoptical waveguide layer 20 containing the liquid crystal material isdriven using the electrodes to change the refractive index anisotropy ofthe liquid crystal. In this manner, the refractive index for the lightpropagating through the optical waveguide layer 20 can be modulated.Generally, a liquid crystal has a birefringence of about 0.1 to 0.2, anda change in refractive index comparable to the birefringence can beobtained by changing the alignment direction of the liquid crystal usingan electric field. In the structure using the birefringent effect of theliquid crystal, the optical waveguide layer 20 of each of the waveguideelements 10 contains the liquid crystal material. The first adjustingelement 60 changes the refractive index anisotropy of the liquid crystalmaterial by applying a voltage to the pair of electrodes 62 to therebychange the refractive index of the optical waveguide layer 20.

The thermooptical effect is a change in the refractive index of amaterial due to a change in its temperature. When the thermoopticaleffect is used for driving, an optical waveguide layer 20 containing athermooptical material may be heated to modulate its refractive index.

FIG. 33 is an illustration showing an example of a structure in which awaveguide element 10 is combined with an adjusting element 60 includinga heater 68 formed of a material having high electrical resistance. Theheater 68 may be disposed near an optical waveguide layer 20. When apower source 66 is turned on, a voltage is applied to the heater 68through wiring lines 64 containing a conductive material, and the heater68 can thereby be heated. The heater 68 may be in contact with theoptical waveguide layer 20. In the present structural example, theoptical waveguide layer 20 of each of the waveguide elements 10 containsa thermooptical material whose refractive index is changed with a changein temperature. The heater 68 included in the first adjusting element 60is disposed in contact with or near the optical waveguide layer 20. Inthe first adjusting element 60, the thermooptical material is heated bythe heater 68 to thereby change the refractive index of the opticalwaveguide layer 20.

The optical waveguide layer 20 itself may be formed of a high-electricresistance material and sandwiched directly between a pair of electrodes62, and a voltage may be applied to the pair of electrodes 62 to heatthe optical waveguide layer 20. In this case, the first adjustingelement 60 includes the pair of electrodes 62 sandwiching the opticalwaveguide layer 20. In the first adjusting element 60, a voltage isapplied to the pair of electrodes 62 to heat the thermooptical material(e.g., a high-electric resistance material) in the optical waveguidelayer 20, and the refractive index of the optical waveguide layer 20 isthereby changed.

The high-electric resistance material used for the heater 68 or theoptical waveguide layer 20 may be a semiconductor or a high-resistivitymetal material. Examples of the semiconductor used include Si, GaAs, andGaN. Examples of the high-resistivity metal material used include iron,nickel, copper, manganese, chromium, aluminum, silver, gold, platinum,and alloys of combinations of these materials. For example, thetemperature dependence dn/dT of the refractive index of Si for lightwith a wavelength of 1,500 nm is 1.87×10⁻⁴ (K⁻¹) (see“Temperature-dependent refractive index of silicon and germanium,” Proc.SPIE 6273, Optomechanical Technologies for Astronomy, 62732J).Therefore, by changing temperature by 500 degrees, the refractive indexcan be changed by about 0.1. When the heater 68 is disposed near theoptical waveguide layer 20 to heat it locally, a large temperaturechange of 500 degrees can be achieved at a relatively fast speed.

The speed of response to change in refractive index by carrier injectionis determined by the life of the carriers. Generally, the life ofcarriers is of the order of nanoseconds (ns), and the speed of responseis about 100 MHz to about 1 GHz.

When an electrooptical material is used, an electric field is applied toinduce polarization of electrons, and the refractive index is therebychanged. The speed of polarization induction is generally very high. Inmaterials such as LiNbO₃ and LiTaO₃, the response time is of the orderof femtoseconds (fs), and this allows high-speed driving at higher than1 GHz.

When a thermooptical material is used, the speed of response to changein refractive index is determined by the rate of temperature increase ordecrease. By heating only a portion in the vicinity of the waveguide, asteep temperature increase is obtained. By turning off the heater afterthe temperature is locally increased, the heat is dissipated to thesurroundings, and the temperature can be steeply reduced. The speed ofresponse can be as high as about 100 KHz.

In the above examples, the first adjusting element 60 changes therefractive indexes of the optical waveguide layers 20 by a constantvalue simultaneously to change the X component of the wave vector of theemitted light. In the refractive index modulation, the amount ofmodulation depends on the properties of the material. To obtain a largeamount of modulation, it is necessary to apply a high electric field orto align the liquid crystal. The direction of the light emitted from thewaveguide elements 10 depends also on the distance between the mirrors30 and 40. Therefore, the thickness of each optical waveguide layer 20may be changed by changing the distance between the mirrors 30 and 40.Next, examples of a structure in which the thickness of the opticalwaveguide layer 20 is changed will be described.

To change the thickness of the optical waveguide layer 20, the opticalwaveguide layer 20 may be formed from an easily deformable material suchas a gas or a liquid. By moving at least one of the mirrors 30 and 40sandwiching the optical waveguide layer 20, the thickness of the opticalwaveguide layer 20 can be changed. In this case, to maintain theparallelism between the upper and lower mirrors 30 and 40, a structurein which the deformation of the mirror 30 or 40 is minimized may beemployed.

FIG. 34 is an illustration showing a structural example in which amirror 30 is held by support members 70 formed of an easily deformablematerial. Each support member 70 may include a thin member or a narrowframe more easily deformable than the mirror 30. In this example, thefirst adjusting element includes an actuator connected to the firstmirror 30 of each waveguide element 10. The actuator changes thedistance between the first mirror 30 and the second mirror 40 to therebychange the thickness of the optical waveguide layer 20. The actuator maybe connected to at least one of the first mirror 30 and the secondmirror 40. The actuator used to drive the mirror 30 may be any ofvarious actuators that utilize, for example, electrostatic force,electromagnetic induction, a piezoelectric material, a shape-memoryalloy, and heat.

In a structure using electrostatic force, the actuator in the firstadjusting element moves the mirror 30 and/or the mirror 40 using anattractive or repulsive force generated between electrodes by theelectrostatic force. Some examples of such a structure will next bedescribed.

FIG. 35 is an illustration showing an example of a structure in whichthe mirror 30 and/or the mirror 40 is moved by an electrostatic forcegenerated between electrodes. In this example, a light-transmittingelectrode 62 (e.g., transparent electrode) is disposed between theoptical waveguide layer 20 and the mirror 30, and anotherlight-transmitting electrode 62 is disposed between the opticalwaveguide layer 20 and the mirror 40. Support members 70 are disposed onboth sides of the mirror 30. One end of each support member 70 is fixedto the mirror 30, and the other end is fixed to an unillustrated casing.When positive and negative voltages are applied to the pair ofelectrodes 62, an attractive force is generated, and the distancebetween the mirrors 30 and 40 is reduced. When the application of thevoltage is stopped, the restoring force of the support members 70holding the mirror 30 allows the distance between the mirrors 30 and 40to be returned to the original length. It is unnecessary that theelectrodes 62 generating the attractive force be provided over theentire mirror surfaces. The actuator in this example includes the pairof electrodes 62. One of the pair of electrodes 62 is fixed to the firstmirror 30, and the other one of the pair of electrodes 62 is fixed tothe second mirror 40. In the actuator, an electrostatic force isgenerated between the pair of electrodes by applying a voltage to theelectrodes to thereby change the distance between the first mirror 30and the second mirror 40. The above-described first driving circuit 110(e.g., FIG. 29) is used to apply the voltage to the electrodes 62.

FIG. 36 is an illustration showing a structural example in whichelectrodes 62 that generate an attractive force are disposed in portionsthat do not impede propagation of light. In this example, it is notnecessary that the electrodes 62 be transparent. As shown in FIG. 36, itis unnecessary that the electrodes 62 fixed to the mirrors 30 and 40 besingle electrodes, and the electrodes 62 may be divided. The distancebetween the mirrors 30 and 40 can be measured by measuring theelectrostatic capacitance between parts of the divided electrodes, andfeedback control can be performed to adjust, for example, theparallelism between the mirrors 30 and 40.

Instead of using the electrostatic force between the electrodes,electromagnetic induction that generates an attractive or repulsiveforce in a magnetic material in a coil may be used to drive the mirror30 and/or the mirror 40.

In an actuator that utilizes a piezoelectric material, a shape-memoryalloy, or deformation by heat, a phenomenon in which a material isdeformed by energy applied from the outside is utilized. For example,lead zirconate titanate (PZT), which is a typical piezoelectricmaterial, expands and contracts when an electric field is applied in itspolarization direction. The use of this piezoelectric material allowsthe distance between the mirrors 30 and 40 to be changed directly.However, since the piezoelectric constant of PZT is about 100 μm/V, theamount of displacement is very small, e.g., about 0.01%, even when anelectric field of 1 V/μm is applied. Therefore, when the piezoelectricmaterial is used, a sufficient mirror moving distance cannot beobtained. However, a structure called unimorph or bimorph may be used toincrease the amount of deformation.

FIG. 37 is an illustration showing an example of a piezoelectric element72 containing a piezoelectric material. Arrows represent the deformationdirections of the piezoelectric element 72, and the sizes of the arrowsrepresent the amounts of deformation. As shown in FIG. 37, since theamounts of deformation of the piezoelectric element 72 depend on thelength of the material, the amount of deformation in the plane directionis larger than the amount of deformation in the thickness direction.

FIG. 38A is an illustration showing a structural example of a supportmember 74 a having a unimorph structure using the piezoelectric element72 shown in FIG. 37. This support member 74 a has a structure in whichone piezoelectric element 72 and one non-piezoelectric element 71 arestacked. This support member 74 a is fixed to at least one of themirrors 30 and 40. Then, by deforming the resulting support member 74 a,the distance between the mirrors 30 and 40 can be changed.

FIG. 38B is an illustration showing an example of a state in which thesupport member 74 a is deformed by applying a voltage to thepiezoelectric element 72. When a voltage is applied to the piezoelectricelement 72, only the piezoelectric element 72 expands in a planedirection, and the entire support member 74 a is thereby bent. Theamount of deformation is larger than that when the non-piezoelectricelement 71 is not provided.

FIG. 39A is an illustration showing a structural example of a supportmember 74 b having a bimorph structure using the piezoelectric element72 shown in FIG. 37. This support member 74 b has a structure in whichtwo piezoelectric elements 72 are stacked with one non-piezoelectricelement 71 disposed therebetween. This support member 74 b is fixed toat least one of the mirrors 30 and 40. Then, by deforming the resultingsupport member 74 b, the distance between the mirrors 30 and 40 can bechanged.

FIG. 39B is an illustration showing a state in which the support member74 a is deformed by applying a voltage to the piezoelectric elements 72on both sides. In the bimorph structure, the deformation direction ofthe upper piezoelectric element 72 is opposite to the deformationdirection of the lower piezoelectric element 72. Therefore, when thebimorph structure is used, the amount of deformation can be larger thanthat using the unimorph structure.

FIG. 40 is an illustration showing an example of an actuator in whichthe support members 74 a shown in FIG. 38A are disposed on both sides ofa mirror 30. By using this piezoelectric actuator, each support member74 a can be deformed, just like a beam is bent, and the distance betweenthe mirrors 30 and 40 can thereby be changed. Instead of the supportmembers 74 a shown in FIG. 38A, the support members 74 b shown in FIG.39A may be used.

The unimorph-type actuator deforms into an arc shape. Therefore, asshown in FIG. 41A, a non-fixed end of the actuator is inclined. If thestiffness of the mirror 30 is low, it is difficult to maintain theparallelism between the mirrors 30 and 40. As shown in FIG. 41B, twounimorph-type support members 74 a with different expansion directionsmay be connected in series. In the support members 74 a in the examplein FIG. 41B, the bending direction of a contracted region is opposite tothe bending direction of an extended region. This can prevent thenon-fixed end from being inclined. By using the above support members 74a, the inclination of the mirrors 30 and 40 can be prevented.

By laminating materials with different thermal expansion coefficients, abendable-deformable beam structure can be obtained, as in the abovecase. Such a beam structure can be obtained by using a shape-memoryalloy. Any of them can be used to control the distance between themirrors 30 and 40.

The distance between the mirrors 30 and 40 can be changed also by thefollowing method. A closed space is used as the optical waveguide layer20, and air or liquid is pumped into or out of the closed space using,for example, a small pump to thereby change the volume of the opticalwaveguide layer 20.

As described above, various structures can be used for the actuator ofthe first adjusting element to change the thickness of the opticalwaveguide layer 20. The thicknesses of the plurality of waveguideelements 10 may be changed separately or together. In particular, whenall the plurality of waveguide elements 10 have the same structure, thedistances between the mirrors 30 and 40 of the waveguide elements 10 arecontrolled uniformly. Therefore, one actuator can be used to drive allthe waveguide elements 10 collectively.

FIG. 42 is an illustration showing an example of a structure in which aplurality of first mirrors held by a support member (i.e., an auxiliarysubstrate) are collectively driven by an actuator. In FIG. 42, oneplate-shaped mirror is used as the second mirror 40. The mirror 40 maybe divided into a plurality of mirrors, as in the above embodiment. Thesupport member 52 is formed of a light-transmitting material, andunimorph-type piezoelectric actuators are disposed on both sides of thesupport member 52.

FIG. 43 is an illustration showing a structural example in which oneplate-shaped first mirror 30 is used for a plurality of waveguideelements 10. In this example, divided second mirrors 40 are provided forrespective waveguide elements 10. As in the examples shown in FIGS. 42and 43, the mirrors 30 or the mirrors 40, or both, of the waveguideelements 10 may be portions of single plate-shaped mirrors. The actuatormay move the plate-shaped mirrors to change the distance between themirrors 30 and 40.

<Refractive Index Modulation for Phase Shifting>

A description will next be given of a structure for adjusting phases ina plurality of phase shifters 80 using the second adjusting element. Thephases in the plurality of phase shifters 80 can be adjusted by changingthe refractive indexes of waveguides 20 a of the phase shifters 80. Therefractive indexes can be changed using the same method as any of theabove-described methods for adjusting the refractive index of theoptical waveguide layer 20 of each of the waveguide elements 10. Forexample, any of the structures and methods for refractive indexmodulation described with reference to FIGS. 32A to 33 can be appliedwithout any modification. Specifically, in the descriptions for FIGS.32A to 33, the waveguide element 10 is replaced with the phase shifter80, the first adjusting element 60 is replaced with the second adjustingelement, the optical waveguide layer 20 is replaced with the waveguide20 a, and the first driving circuit 110 is replaced with the seconddriving circuit 210. Therefore, the detailed description of therefractive index modulation in the phase shifter 80 will be omitted.

The waveguide 20 a of each of the phase shifters 80 contains a materialwhose refractive index is changed when a voltage is applied ortemperature is changed. The second adjusting element changes therefractive index of the waveguide 20 a of each of the phase shifters 80by applying a voltage to the waveguide 20 a or changing the temperatureof the waveguide 20 a. In this manner, the second adjusting element canchange the phase differences between light beams propagating from theplurality of phase shifters 80 to the plurality of waveguide elements10.

Each phase shifter 80 may be configured such that the phase of light canbe shifted by at least 2π when the light passes through. When the amountof change in the refractive index per unit length of the waveguide 20 aof the phase shifter 80 is small, the length of the waveguide 20 a maybe increased. For example, the size of the phase shifter 80 may beseveral hundreds of micrometers (μm) to several millimeters (mm) or maybe lager for some cases. However, the length of each waveguide element10 may be several tens of micrometers to several tens of millimeters.

<Structure for Synchronous Driving>

In the present embodiment, the first adjusting element drives theplurality of waveguide elements 10 such that light beams emitted fromthe waveguide elements 10 are directed in the same direction. To directthe light beams emitted from the plurality of waveguide elements 10 inthe same direction, driving units are provided for their respectivewaveguide elements 10 and driven synchronously.

FIG. 44 is an illustration showing an example of a structure in whichcommon wiring lines 64 are led from electrodes 62 of the waveguideelements 10. FIG. 45 is an illustration showing an example of astructure in which the wiring lines 64 and some of the electrodes 62 areshared. FIG. 46 is an illustration showing an example of a structure inwhich common electrodes 62 are provided for a plurality of waveguideelements 10. In FIGS. 44 to 46, each straight arrow indicates the inputof light. With the structures shown in FIGS. 44 to 46, the wiring fordriving the waveguide array 10A can be simplified.

With the structures in the present embodiment, two-dimensional opticalscanning can be performed using a simple device structure. For example,when a waveguide array including N waveguide elements 10 is driven in asynchronous manner using independent driving circuits, N drivingcircuits are necessary. However, when common electrodes or wiring linesare used in an ingenious manner, only one driving circuit may be usedfor operation.

When the phase shifter array 80A is disposed upstream of the waveguidearray 10A, additional N driving circuits are necessary to drive thephase shifters 80 independently. However, as shown in the example inFIG. 31, by arranging the phase shifters 80 in a cascaded manner, onlyone driving circuit may be used for driving. Specifically, with thestructures in the present disclosure, a two-dimensional optical scanningoperation can be implemented by using 2 to 2N driving circuits. Thewaveguide array 10A and the phase shifter array 80A may be operatedindependently, so that their wiring lines can be easily arranged with nointerference.

<Production Method>

The waveguide array, the phase shifter array 80A, and the waveguidesconnecting them can be produced by a process capable of high-precisionfine patterning such as a semiconductor process, a 3D printer,self-organization, or nanoimprinting. With such a process, all necessarycomponents can be integrated in a small area.

In particular, the use of a semiconductor process is advantageousbecause very high processing accuracy and high mass productivity can beachieved. When the semiconductor process is used, various materials canbe deposited on a substrate using vacuum evaporation, sputtering, CVD,application, etc. Fine patterning can be achieved by photolithographyand an etching process. For example, Si, SiO₂, Al₂O₃, AlN, SiC, GaAs,GaN, etc. can be used as the material of the substrate.

<Modifications>

Modifications of the present embodiment will next be described.

FIG. 47 is an illustration schematically showing an example of astructure in which waveguides are integrated into a small array while alarge arrangement area is allocated for the phase shifter array 80A.With this structure, even when the change in the refractive index of thematerial forming the waveguides of the phase shifters 80 is small, asufficient phase shift amount can be ensured. When each phase shifter 80is driven using heat, the influence on its adjacent phase shifters 80can be reduced because large spacing can be provided between them.

FIG. 48 is an illustration showing a structural example in which twophase shifter arrays 80Aa and 80Ab are disposed on respective sides ofthe waveguide array 10A. In the optical scanning device 100 in thisexample, two optical dividers 90 a and 90 b and the two phase shifterarrays 80Aa and 80Ab are disposed on respective sides of the waveguidearray 10A. Dotted straight arrows in FIG. 48 indicate light beamspropagating through the optical dividers 90 a and 90 b and the phaseshifters 80 a and 80 b. The phase shifter array 80Aa and the opticaldivider 90 a are connected to one side of the waveguide array 10A, andthe phase shifter array 80Ab and the optical divider 90 b are connectedto the other side of the waveguide array 10A. The optical scanningdevice 100 further includes an optical switch 92 that switches betweensupply of light to the optical divider 90 a and supply of light to theoptical divider 90 b. The optical switch 92 allows switching between thestate in which light is inputted to the waveguide array 10A from theleft side in FIG. 48 and the state in which light is inputted to thewaveguide array 10A from the right side in FIG. 48.

The structure in this modification is advantageous in that the range ofscanning in the X direction with the light emitted from the waveguidearray 10A can be increased. In a structure in which light is inputted tothe waveguide array 10A from one side, the direction of the light can bechanged from the front direction (the +Z direction) toward one of the +Xdirection and the −X direction by driving the waveguide elements 10. Inthe present modification, when the light is inputted from the leftoptical divider 90 a in FIG. 48, the direction of the light can bechanged from the front direction toward the +X direction. When the lightis inputted from the right optical divider 90 b in FIG. 48, thedirection of the light can be changed from the front direction towardthe −X direction. Specifically, in the structure in FIG. 48, thedirection of the light can be changed in both the left and rightdirections in FIG. 48 as viewed from the front. Therefore, the scanningangle range can be larger than that when the light is inputted from oneside. The optical switch 92 is controlled by an electric signal from anunillustrated control circuit (e.g., a microcontroller unit). In thisstructural example, all the elements can be driven and controlled usingelectric signals.

In all the waveguide arrays in the above description, the arrangementdirection of the waveguide elements 10 is orthogonal to the extendingdirection of the waveguide elements 10. However, it is unnecessary thatthese directions be orthogonal to each other. For example, a structureshown in FIG. 49A may be used. FIG. 49A shows a structural example of awaveguide array in which an arrangement direction d1 of waveguideelements 10 is not orthogonal to an extending direction d2 of thewaveguide elements 10. In this example, the light-emission surfaces ofthe waveguide elements 10 may not be in the same plane. Even with thisstructure, the emission direction d3 of light can be changedtwo-dimensionally by appropriately controlling the waveguide elements 10and the phase shifters.

FIG. 49B shows a structural example of a waveguide array in whichwaveguide elements 10 are arranged at non-regular intervals. Even whenthis structure is employed, two-dimensional scanning can be performed byappropriately setting the phase shift amounts by the phase shifters.Also in the structure in FIG. 49B, the arrangement direction d1 of thewaveguide array may not be orthogonal to the extending direction d2 ofthe waveguide elements 10.

<Embodiment in which First and Second Waveguides are Disposed onSubstrate>

Next, an embodiment of an optical scanning device in which first andsecond waveguides are disposed on a substrate will be descried.

The optical scanning device in the present embodiment includes: firstwaveguides; second waveguides connected to the first waveguides; and asubstrate that supports the first and second waveguides. Morespecifically, the optical scanning device includes: a plurality ofwaveguide units arranged in a first direction; and the substrate thatsupports the plurality of waveguide units. Each of the plurality ofwaveguide units includes a first waveguide and a second waveguide. Thesecond waveguide is connected to the first waveguide and propagateslight in a second direction intersecting the first direction. Thesubstrate supports the first waveguide and the second waveguide of eachof the waveguide units.

The second waveguide corresponds to the reflective waveguide in theembodiment described above. Specifically, the second waveguide includes:a first mirror including a multilayer reflective film; a second mirrorincluding a multilayer reflective film facing the multilayer reflectivefilm of the first mirror; and an optical waveguide layer that is locatedbetween the first and second mirrors and propagates light inputted tothe first waveguide and transmitted therethrough. The first mirror has ahigher light transmittance than the second mirror and allows part of thelight propagating through the optical waveguide layer to be emitted tothe outside of the optical waveguide layer. The optical scanning devicefurther includes an adjusting element that changes at least one of therefractive index and thickness of the optical waveguide layer of each ofthe second waveguides to thereby change the direction of light emittedfrom the each of the second waveguides.

In the present embodiment, the first and second waveguides are disposedon one substrate, so that the first waveguides 1 and the secondwaveguides 10 can be easily aligned with each other. In addition,positional displacement between the first and second waveguides due tothermal expansion is reduced. Therefore, light beams can be efficientlyintroduced from the first waveguides to the second waveguides.

Each optical waveguide layer may contain a material whose refractiveindex for the light propagating through the optical waveguide layer ischanged when a voltage is applied. In this case, the adjusting elementchanges the refractive index of the optical waveguide layer by applyinga voltage to the optical waveguide layer. In this manner, the adjustingelement changes the direction of the light emitted from each secondwaveguide.

At least part of each first waveguide may have the function as the phaseshifter described above. In this case, a mechanism that modulates therefractive index of the first waveguide is installed in the firstwaveguide. The optical scanning device may further include a secondadjusting element that modulates the refractive index of at least apartial region of each first waveguide. The second adjusting element maybe a heater disposed in the vicinity of the first waveguide. Therefractive index of at least the partial region of the first waveguidecan be changed by heat generated by the heater. In this manner, thephases of light beams inputted from the first waveguides to the secondwaveguides are adjusted. As described above, various structures can beused to adjust the phases of the light beams inputted from the firstwaveguides to the second waveguides. Any of these structures may beused.

The phase shifters may be disposed outside of the first waveguides. Inthis case, each first waveguide is disposed between a correspondingexternal phase shifter and a corresponding waveguide element (secondwaveguide). No clear boundary may be present between the phase shifterand the first waveguide. For example, the phase shifter and the firstwaveguide may share components such as a waveguide and the substrate.

Each first waveguide may be a general waveguide that utilizes totalreflection of light or may be a reflective waveguide. Thephase-modulated light beam passes through the first waveguide and isintroduced into the corresponding second waveguide.

The embodiment of the optical scanning device in which the first andsecond waveguides are disposed on the substrate will be described inmore detail. In the following description, the optical scanning deviceincludes a plurality of waveguide units. The optical scanning device mayinclude only one waveguide unit. Specifically, an optical scanningdevice including only one pair of first and second waveguides isincluded in the scope of the present disclosure.

FIG. 50A is an illustration schematically showing the optical scanningdevice in the present embodiment. This optical scanning device includesa plurality of waveguide units arranged in the Y direction and asubstrate 50 that supports the plurality of waveguide units. Each of thewaveguide units includes a first waveguide 1 and a second waveguide 10.The substrate 50 supports the first waveguide 1 and the second waveguide10 of each of the waveguide units.

The substrate 50 extends along the XY plane. The upper and lowersurfaces of the substrate 50 are disposed approximately parallel to theXY plane. The substrate 50 may be formed of a material such as glass Si,SiO₂, GaAs, or GaN.

A first waveguide array 1A includes a plurality of the first waveguides1 arranged in the Y direction. Each of the first waveguides 1 has astructure extending in the X direction. A second waveguide array 10Aincludes a plurality of the second waveguides 10 arranged in the Ydirection. Each of the second waveguides 10 has a structure extending inthe X direction.

FIG. 50B is a cross-sectional view of the optical scanning device in theXZ plane shown by one of broken lines in FIG. 50A. First and secondwaveguides 1 and 10 are disposed on the substrate 50. A second mirror 40extends in a region between an optical waveguide layer 20 and thesubstrate 50 and between the first waveguide 1 and the substrate 50. Thefirst waveguide 1 is, for example, a general waveguide that uses totalreflection of light. One example of the general waveguide is a waveguideformed of a semiconductor such as Si or GaAs. The second waveguide 10includes the optical waveguide layer 20 and first and second mirrors 30and 40. The optical waveguide layer 20 is located between the firstmirror 30 and the second mirror 40 facing each other. The opticalwaveguide layer 20 propagates light inputted to the first waveguide andtransmitted therethrough.

The optical waveguide layer 20 in the present embodiment contains amaterial whose refractive index for the light beam propagating throughthe optical waveguide layer 20 is changed when a voltage is applied. Theadjusting element includes a pair of electrodes. The pair of electrodesincludes a lower electrode 62 a and an upper electrode 62 b. The lowerelectrode 62 a is disposed between the optical waveguide layer 20 andthe second mirror 40. The upper electrode 62 b is disposed between theoptical waveguide layer 20 and the first mirror 30. The adjustingelement in the present embodiment changes the refractive index of theoptical waveguide layer 20 by applying a voltage to the pair ofelectrodes 62 a and 62 b. In this manner, the adjusting element changesthe direction of the light emitted from each second waveguide 10. Eachof the electrodes 62 a and 62 b may be in contact with the opticalwaveguide layer 20 as shown in FIG. 50B or may not be in contact withthe optical waveguide layer 20.

In the structural example in FIG. 50B, the second mirror 40 is stackedon the substrate 50 to form a common support, and other structures aredisposed on the support. Specifically, a stack including the firstwaveguides 1, the first electrode 62 a, the optical waveguide layers 20,the second electrodes 62 b, and the first mirrors 30 is formed on theintegrally formed support. Since the common support is used, the firstwaveguides 1 and the optical waveguide layers 20 are easily aligned witheach other during production. In addition, positional displacement ofconnection portions between the first waveguides 1 and the opticalwaveguide layer 20 due to thermal expansion can be reduced. The supportis, for example, a support substrate.

FIG. 50C is a cross-sectional view of the optical scanning device in theYZ plane shown by the other one of the broken lines in FIG. 50A. In thisexample, the second mirror 40 is shared by the plurality of secondwaveguides 10. Specifically, the second mirror 40 is not divided, andthis non-divided second mirror 40 is used for the plurality of secondwaveguides 10. Similarly, the lower electrode 62 a is shared by theplurality of second waveguides 10. This allows the production process tobe simplified.

In the plurality of second waveguides 10, the optical waveguide layers20 are separated from each other. The upper electrodes 62 b areseparated from each other, and the first mirrors 30 are separated fromeach other. In this manner, each optical waveguide layer 20 canpropagate light in the X direction. The upper electrodes 62 b and thefirst mirrors 30 may be a single non-divided upper electrode 62 and asingle non-divided first mirror 30, respectively.

Modifications of the optical scanning device in the present embodimentwill be described. In the following modifications, repeated descriptionof the same components will be omitted.

FIG. 51A is an illustration showing a structural example in which adielectric layer 51 is disposed between the second mirror 40 and thewaveguide 1. The optical scanning device in this example furtherincludes the dielectric layer 51 extending between the second mirror 40and the first waveguide 1. The dielectric layer 51 serves as anadjustment layer for adjusting the height level of the first waveguide 1relative to the height level of the optical waveguide layer 20.Hereinafter, the dielectric layer 51 is referred to as the adjustmentlayer 51. By adjusting the thickness of the adjustment layer 51 in the Zdirection, the coupling efficiency of light from the first waveguide 1to the optical waveguide layer 20 can be increased. The adjustment layer51 serves also as a spacer that prevents the guided light in the firstwaveguide 1 from being absorbed, scattered, and reflected by the secondmirror 40. The first waveguide 1 propagates light by total reflection.Therefore, the adjustment layer 51 is formed of a transparent materialhaving a lower refractive index than the first waveguide 1. For example,the adjustment layer 51 may be formed of a dielectric material such asSiO₂.

Another dielectric layer serving as a protective layer may be disposedon the first waveguide.

FIG. 51B is an illustration showing a structural example in which asecond dielectric layer 61 is disposed on the first waveguide 1. Asdescribed above, the optical scanning device may further include thesecond dielectric layer 61 that covers at least part of the firstwaveguide 1. The second dielectric layer 61 is in contact with the firstwaveguide 1 and is formed of a transparent material having a lowerrefractive index than the first waveguide 1. The second dielectric layer61 serves also as the protective layer that prevents particles and dustfrom adhering to the first waveguide 1. This can reduce loss of theguided light in the first waveguide 1. Hereinafter, the seconddielectric layer 61 is referred to as the protective layer 61.

The first waveguide 1 shown in FIG. 51B functions as a phase shifter.The optical scanning device further includes a second adjusting elementthat modulates the refractive index of the first waveguide 1 to therebychange the phase of the light introduced into the optical waveguidelayer 20. When the first waveguide 1 contains a thermooptical material,the second adjusting element includes a heater 68. The second adjustingelement modulates the refractive index of the first waveguide 1 usingheat generated by the heater 68.

A wiring material such as a metal contained in the heater 68 can absorb,scatter, or reflect light. The protective layer 61 keeps the heater 68at a distance from the first waveguide 1 to thereby reduce loss of theguided light in the first waveguide 1.

The protective layer 61 may be formed of the same material as thematerial (e.g., SiO₂) of the adjustment layer 51. The protective layer61 may cover not only the first waveguide 1 but also at least part ofthe second waveguide 10. In this case, at least part of the first mirror30 is covered with the protective layer 61. The protective layer 61 maycover only the second waveguide 10. When the protective layer 61 isformed of a transparent material, the light emitted from the secondwaveguide 10 passes through the protective layer 61. This allows theloss of light to be small.

FIG. 52 is an illustration showing a structural example in which thesecond mirror 40 is not disposed in a region between the first waveguide1 and the substrate 50. The adjustment layer 51 in this example extendsin the region between the first waveguide 1 and the substrate 50. Theadjustment layer 51 is in contact with the first waveguide 1 and thesubstrate 50. Since the second mirror 40 is not present below the firstwaveguide 1, the guided light in the first waveguide 1 is not influencedby the second mirror 40.

FIG. 53 is an illustration showing a structural example in which,between the first waveguide 1 and the substrate 50, the second mirror 40is thinner than the second mirror 40 in the structural example in FIG.51B. The second mirror 40 may have a portion disposed between the firstwaveguide 1 and the substrate 50 and having a smaller thickness than aportion disposed between the second waveguide 10 and the substrate 50,as in this example. The adjustment layer 51 is disposed between thefirst waveguide 1 and the second mirror 40. In this structure, theguided light in the first waveguide 1 is less influenced by the secondmirror 40. In the example in FIG. 53, a step is formed by the secondmirror 40 at the junction between the first waveguide 1 and the opticalwaveguide layer 20, but the height of the step is smaller than that inthe example in FIG. 52. Therefore, the second mirror 40 can be moreeasily processed.

The thickness of the second mirror 40 may vary along the waveguide 1.Such an example will next be described.

FIG. 54A is an illustration showing a structural example in which thethickness of the second mirror 40 varies gradually. Between the firstwaveguide 1 and the substrate 50, the thickness of the second mirror 40varies along the first waveguide 1.

In the example in FIG. 54A, the second mirror 40 is not present below aleft portion of the first waveguide 1. The left portion of the firstwaveguide 1 is located lower than the optical waveguide layer 20. Thesecond mirror 40 is present below a right portion of the first waveguide1, i.e., its portion connected to the optical waveguide layer 20. Theright portion of the first waveguide 1 is located at about the sameheight as the optical waveguide layer 20. By adjusting the thickness ofthe protective layer 61, the upper surface of the protective layer 61can be made flat.

In the structural example in FIG. 54A, the heater 68 disposed on theprotective layer 61 is sufficiently spaced apart from the firstwaveguide 1. Therefore, the guided light in the first waveguide 1 isless influenced by the wiring of the heater 68. The loss of the guidedlight in the first waveguide 1 can thereby be reduced.

FIG. 54B is an illustration showing a structural example in which theupper electrode 62 b, the first mirror 30, and a second substrate 50Care disposed so as to extend over the protective layer 61 of the firstwaveguide 1 and the optical waveguide layer 20 of the second waveguide10. FIG. 54C is an illustration showing part of a production process inthe structural example in FIG. 54B.

In the example in FIG. 54B, a structural body including the upperelectrode 62 b, the first mirror 30, and the second substrate 50C(hereinafter referred to as an “upper structural body”) and a structuralbody lower than the upper electrode 62 b (hereinafter referred to as a“lower structural body”) are produced separately.

To produce the lower structural body, the second mirror 40 having aninclination is first formed on the first substrate 50. The adjustmentlayer 51, a layer of the waveguide 1, and the protective layer 61 areformed in this order on a portion of the second mirror 40 that includesthe inclination. The lower electrode 62 a and the optical waveguidelayer 20 are formed on a flat portion of the second mirror 40.

The upper structural body is produced by stacking the first mirror 30and the upper electrode 62 b in this order on the second substrate 50C.As shown in FIG. 54C, the upper structural body is turned upside downand then laminated onto the lower structural body. With the aboveproduction method, it is unnecessary to precisely align the firstwaveguide 1 and the second waveguide 10 with each other.

The upper surface of the protective layer 61, i.e., its surface oppositeto the surface in contact with the first waveguide 1, is lower than theupper surface of the optical waveguide layer 20 of the second waveguide10. The upper surface of the heater 68 on the first waveguide 1 is atabout the same level as the upper surface of the optical waveguide layer20 of the second waveguide 10. In this case, the upper structural bodyand the lower structural body can be laminated together with no step.The upper structural body may be formed by, for example, vapordeposition or sputtering.

FIG. 55 is an illustration showing a YZ-plane cross section of aplurality of second waveguides 10 in an optical scanning device havingthe structure shown in FIG. 54B. In this example, the plurality ofsecond waveguides 10 share the first mirror 30, the second mirror 40,and the electrodes 62 a and 62 b. A plurality of optical waveguidelayers 20 are disposed between the common electrodes 62 a and 62 b.Regions between the plurality of optical waveguide layers 20 serve asspacers 73. The spacers 73 are, for example, air (or a vacuum) or atransparent material such as SiO₂, TiO₂, Ta₂O₅, SiN, or AlN. When thespacers 73 are formed of a solid material, the upper structural body canbe formed by, for example, vapor deposition or sputtering. Each spacer73 may be in direct contact with two adjacent optical waveguide layers20.

It is unnecessary that the first waveguides 1 be general waveguides thatuse total reflection of light. For example, the first waveguides 1 maybe reflective waveguides similar to the second waveguides 10.

FIG. 56 is an illustration showing a structural example in which thefirst waveguide 1 and the second waveguide 10 are reflective waveguides.The first waveguide 1 is sandwiched between two opposed multilayerreflective films 3 and 40. The principle of light propagation throughthe first waveguide 1 is the same as the principle of light propagationthrough the second waveguide 10. When the thickness of the multilayerreflective film 3 is sufficiently large, no light is emitted from thefirst waveguide 1.

In the structural example in FIG. 56, the coupling efficiency of lightcan be increased by optimizing the connection conditions of the tworeflective waveguides, as described above with reference to FIGS. 20,21, etc. The optimization allows light to be efficiently introduced fromthe first waveguide 1 to the second waveguide 10.

Next, modifications of the arrangement of the pair of electrodes 62 aand 62 b will be described. In the examples in FIGS. 50A to 56, the pairof electrodes 62 a and 62 b are in contact with the optical waveguidelayer 20 of the second waveguide 10. In the examples in FIGS. 50C and55, the plurality of second waveguides 10 shares one or both of theelectrodes 62 a and 62 b. However, the structure of the electrodes 62 aand 62 b is not limited to the above structures.

FIG. 57 is an illustration showing a structural example in which theupper electrode 62 b is disposed on the upper surface of the firstmirror 30 and the lower electrode 62 a is disposed on the lower surfaceof the second mirror 40. The first mirror 30 is disposed between theupper electrode 62 b and the optical waveguide layer 20. The secondmirror 40 is disposed between the lower electrode 62 a and the opticalwaveguide layer 20. As shown in this example, the pair of electrodes 62a and 62 b may sandwich the optical waveguide layer 20 indirectlythrough the first and second mirrors 30 and 40.

In the example in FIG. 57, the lower electrode 62 a extends to the firstwaveguide 1 side. When a wiring line is led from the lower electrode 62a, a space below the first waveguide 10 can be used. Therefore, thedesign flexibility of the wiring line is increased.

In this example, the pair of electrodes 62 a and 62 b are not in contactwith the optical waveguide layer 20. The guided light in the opticalwaveguide layer 20 is less influenced by absorption, scattering, andreflection by the pair of electrodes 62 a and 62 b. Therefore, the lossof the guided light in the optical waveguide layer 20 can be reduced.

FIG. 58 is a cross-sectional view showing another modification. In thisexample, the first waveguide 1 is separated into a first portion 1 a anda second portion 1 b. The first portion 1 a is located at a lowerposition and spaced apart from the second waveguide 10. The secondportion 1 b is located at a higher position and connected to the opticalwaveguide layer 20 of the second waveguide 10. The first portion 1 a andthe second portion 1 b overlap each other when viewed in the +Zdirection. The first portion 1 a and the second portion 1 b areapproximately parallel to each other and extend in the X direction. Inthis example, the adjustment layer 51 is also separated into twoportions 51 a and 51 b. The first portion 51 a of the adjustment layeris disposed between the first portion 1 a of the first waveguide and thelower electrode 62 a. The second portion 51 b of the adjustment layer isdisposed between the second portion 1 b of the first waveguide and thesecond mirror 40. The protective layer 61 is disposed on the firstportion 1 a and second portion 1 b of the first waveguide. A part of thefirst portion 1 a of the first waveguide faces a part of the secondportion 1 b of the first waveguide through the protective layer 61. Thearrangement of the electrodes 62 a and 62 b is the same as thearrangement in FIG. 57.

In the structure shown in FIG. 58, the spacing between the first portion1 a and second portion 1 b of the first waveguide, i.e., their distancein the Z direction, is equal to or less than the wavelength of light inthe waveguide. In this case, the light can be propagated from the firstportion 1 a to the second portion 1 b through evanescent coupling. Inthis example, unlike the example in FIG. 54A, it is unnecessary tochange the thickness of the second mirror 40 along the first waveguides1 a and 1 b.

FIG. 59 is an illustration showing a structural example in whichelectrodes 62 are disposed between adjacent optical waveguide layers 20.The adjusting element in this example includes the electrodes 62 andapplies positive and negative voltages (denoted by “+” and “−” in thefigure) to the electrodes in an alternate manner. In this manner,electric fields in the left-right direction in FIG. 59 can be generatedin the optical waveguide layers 20.

In the example in FIG. 59, two electrodes 62 adjacent in the Y directionare in contact with at least part of an optical waveguide layer 20disposed therebetween. The area of contact between the optical waveguidelayer 20 and each electrode 62 is small. Therefore, even when theelectrodes 62 are formed of a material that absorbs, scatters, orreflects light, the loss of the guided light in the optical waveguidelayer 20 can be reduced.

In the structural examples in FIGS. 50A to 59, light used for scanningis emitted through the first mirror 30. The light used for scanning maybe emitted through the second mirror 40.

FIG. 60 is an illustration showing a structural example in which thefirst mirror 30 is thick and the second mirror 40 is thin. In theexample in FIG. 60, light passes through the second mirror 40 and isemitted from the substrate 50 side. The substrate 50 in this example isformed of a light-transmitting material. When the light emitted from thesubstrate 50 is used for scanning, the design flexibility of the opticalscanning device increases.

<Discussion about Width of Mirrors>

FIG. 61 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing a structural example of a waveguidearray 10A in an embodiment in which a plurality of waveguide elements 10are arranged in the Y direction. In the structural example in FIG. 61,the width of the first mirrors 30 in the Y direction is larger than thewidth of the optical waveguide layers 20. The plurality of waveguideelements 10 share one second mirror 40. In other words, the secondmirror 40 in each waveguide element 10 is a part of one integratedmirror. Each first mirror 30 has portions protruding in the Y directionfrom edge surfaces of a corresponding optical waveguide layer 20. The Ydirection size of the protruding portions is denoted by y₁. The distancefrom an edge surface of the optical waveguide layer 20 in the Ydirection is denoted by y. y=0 corresponds to the edge surface of theoptical waveguide layer 20.

When the guided light propagates through the optical waveguide layer 20in the X direction, evanescent light leaks from the optical waveguidelayer 20 in the Y direction. The intensity I of the evanescent light inthe Y direction is represented by the following formula.

$\begin{matrix}{I = {I_{0}{\exp \left( {- \frac{y}{y_{d}}} \right)}}} & (23)\end{matrix}$

Here, y_(d) satisfies the following formula.

$\begin{matrix}{y_{d} = \frac{\lambda}{4\pi \sqrt{{n_{w}^{2}\sin^{2}\theta_{in}} - n_{low}^{2}}}} & (24)\end{matrix}$

Here, I₀ is the intensity of the evanescent light at y=0. The totalreflection angle θ_(in) is shown in FIG. 11. At y=y_(d), the intensityof the evanescent light is I₀ times 1/e. Here, e is the base of naturallogarithm.

For the sake of simplicity, the guided light in the optical waveguidelayer 20 is approximated as a ray of light, as shown in FIG. 11. Asshown in the structural example in FIG. 61, when no first mirror 30 ispresent in a region satisfying y>y₁, light leakage, or light loss(L_(loss)), per reflection of the guided light at y=0 is represented bythe following formula.

$\begin{matrix}{L_{loss} = {\frac{\int_{y_{1}}^{\infty}{I_{0}{\exp \left( {- \frac{y}{y_{d}}} \right)}{dy}}}{\int_{0}^{\infty}{I_{0}{\exp \left( {- \frac{y}{y_{d}}} \right)}{dy}}} = {\exp \left( {- \frac{y_{1}}{y_{d}}} \right)}}} & (25)\end{matrix}$

As shown in formula (4), to set the divergence angle θ_(div) of lightemitted from the waveguide element 10 to 0.1° or less, it is desirablethat the propagation length L in the waveguide element 10 in the Xdirection is 1 mm or more. Let the width of the optical waveguide layer20 in the Y direction be “a.” Then the number of total reflections inthe ±Y directions in FIG. 11 is 1,000/(a·tan θ_(in)) or more. When a=1μm and θ_(in)=45°, the number of total reflections is 1,000 or more.Using formula (25) representing the light loss per reflection, the lightloss after β reflections is represented by the following formula.

$\begin{matrix}{L_{loss}^{(\beta)} = {1 - \left\{ {1 - {\exp \left( {- \frac{y_{1}}{y_{d}}} \right)}} \right\}^{\beta}}} & (26)\end{matrix}$

FIG. 62 is a graph showing the relation between the ratio of light loss(L^((β)) loss) and y₁ when β=1,000. The vertical axis represents theratio of light loss, and the horizontal axis represents y₁. As shown inFIG. 62, to reduce the ratio of light loss to 50% or less, it is onlynecessary that y₁≥7y_(d). Similarly, to reduce the ratio of light lossto 10% or less, it is only necessary that y₁≥9y_(d). To reduce the ratioof light loss to 1% or less, it is only necessary that y₁≥11y_(d).

As shown by formula (25), in principle, the light loss can be reduced byincreasing y₁. However, the light loss does not become zero.

FIG. 63 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing another example of the waveguide array10A in the present embodiment in which the plurality of waveguideelements 10 are arranged in the Y direction. In the structural examplein FIG. 63, the plurality of waveguide elements 10 share the first andsecond mirrors 30 and 40. In other words, the first mirror 30 of eachwaveguide element 10 is a part of one integrated mirror, and the secondmirror 40 of each waveguide element 10 is a part of one integratedmirror. In principle, this can minimize the light loss.

Next, leakage of evanescent light from each optical waveguide layer 20was numerically computed for each of the structural examples in FIGS. 10and 63, and the results were compared.

FIG. 64A is a graph showing the results of computations of an electricfield intensity distribution in the structural example in FIG. 10. FIG.64B is a graph showing the results of computations of an electric fieldintensity distribution in the structural example in FIG. 63. FemSimavailable from Synopsys was used for the numerical computations. InFIGS. 64A and 64B, the width of the optical waveguide layer 20 in the Ydirection is 1.5 μm, and the thickness of the optical waveguide layer 20in the Z direction is 1 μm. The wavelength of the light is 1.55 μm.n_(w) is 1.68, and n_(low) is 1.44. This combination of n_(w) andn_(low) corresponds to the case in which, for example, a liquid crystalmaterial contained in the optical waveguide layer 20 is enclosed by SiO₂spacers 73.

As can be seen from FIG. 64A, in the structural example in FIG. 10,evanescent light leaks from regions in which no first mirror 30 ispresent. However, as can be seen from FIG. 64B, in the structuralexample in FIG. 63, the leakage of evanescent light is negligible. InFIGS. 64A and 64B, when the guided light propagates in the X direction,the intensity of the guided light decreases because of light emissionfrom the first mirror 30 and leakage of evanescent light. The Xdirection propagation length of the guided light at which the intensityof the guided light is reduced by a factor of e was computed. Thepropagation length of the light in FIG. 64A was 7.8 μm, and thepropagation length in FIG. 64B was 132 μm.

In the present embodiment, the spacers 73 may be formed of two or moredifferent mediums.

FIG. 65 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing a structural example in the presentembodiment in which the spacers 73 include spacers 73 a and 73 b havingdifferent refractive indexes. In the structural example in FIG. 65, therefractive index n_(low1) of the spacers 73 a adjacent to the opticalwaveguide layers 20 is higher than the refractive index n_(low2) of thespacers 73 b not adjacent to the optical waveguide layers 20(n_(low1)>n_(low2)). For example, when the optical waveguide layers 20contain a liquid crystal material, SiO₂ may be used for the spacers 73 ain order to enclose the liquid crystal material. The spacers 73 b may beair. When the refractive index n_(low2) of the spacers 73 b is low,leakage of evanescent light from the optical waveguide layers 20 can besuppressed.

FIG. 66 is a cross-sectional view of an optical scanning device in theYZ plane, schematically showing a structural example of a waveguideelement 10 in a modification of the present embodiment. In thestructural example in FIG. 66, the optical waveguide layer 20 has atrapezoidal cross section in the YZ plane. The first mirror 30 isdisposed not only on the upper side of the optical waveguide layer 20but also on its left and right sides. In this manner, light leakage fromthe left and right sides of the optical waveguide layer 20 can beprevented.

Next, the materials of the optical waveguide layers 20 and the spacers73 will be described.

In the structural examples in FIGS. 61, 63, and 65, the refractive indexn_(w) of the optical waveguide layers 20 and the refractive indexn_(low) of the spacers 73 satisfy the relation n_(w)>n_(low).Specifically, the spacers 73 contain a material having a lowerrefractive index than the material of the optical waveguide layers 20.For example, when the optical waveguide layers 20 contain anelectrooptical material, the spacers 73 may contain a transparentmaterial such as SiO₂, TiO₂, Ta₂O₅, SiN, AlN, or air. When the opticalwaveguide layers 20 contain a liquid crystal material, the spacers 73may contain SiO₂ or air. By sandwiching the optical waveguide layers 20between a pair of electrodes and applying a voltage to the electrodes,the refractive index of the optical waveguide layers 20 containing anelectrooptical material or a liquid crystal material can be changed. Inthis manner, the emission angle of the light emitted from each firstmirror 30 can be changed. The detailed driving method etc. of theoptical scanning device when the optical waveguide layers 20 contain aliquid crystal material or an electrooptical material are as describedabove.

The electrooptical material used may be any of the following compounds.

-   -   KDP (KH₂PO₄) crystals such as KDP, ADP (NH₄H₂PO₄), KDA        (KH₂AsO₄), RDA (RbH₂PO₄), and ADA (NH₄H₂AsO₄)    -   Cubic crystal materials such as KTN, BaTiO₃, SrTiO₃Pb₃MgNb₂O₉,        GaAs, CdTe, and InAs    -   Tetragonal crystal materials such as LiNbO₃ and LiTaO₃    -   Zincblende materials such as ZnS, ZnSe, ZnTe, GaAs, and CuCl    -   Tungsten bronze materials such as KLiNbO₃, SrBaNb₂O₆, KSrNbO,        BaNaNbO, and Ca₂Nb₂O₇

The liquid crystal material used may be, for example, a nematic liquidcrystal. The molecular structure of the nematic liquid crystal is asfollows.

R1-Ph1-R2-Ph2-R3

Here, R1 and R3 each represent an amino group, a carbonyl group, acarboxyl group, a cyano group, an amine group, a nitro group, a nitrilegroup, or an alkyl chain. Ph1 and Ph2 each represent an aromatic groupsuch as a phenyl group or a biphenyl group. R2 represents a vinyl group,a carbonyl group, a carboxyl group, a diazo group, or an azoxy group.

The liquid crystal is not limited to the nematic liquid crystal. Forexample, a smectic liquid crystal may be used. When the liquid crystalis a smectic liquid crystal, the smectic liquid crystal may exhibit, forexample, a smectic C (SmC) phase. The liquid crystal exhibiting thesmectic C (SmC) phase may be, for example, a ferroelectric liquidcrystal exhibiting a chiral smectic (SmC*) phase in which the liquidcrystal molecules have a chiral center (an asymmetric carbon atom).

The molecular structure of the SmC* phase is represented as follows.

R1 and R4 are each an amino group, a carbonyl group, a carboxyl group, acyano group, an amine group, a nitro group, a nitrile group, or an alkylchain. Ph1 and Ph2 are each an aromatic group such as a phenyl group ora biphenyl group. R2 is a vinyl group, a carbonyl group, a carboxylgroup, a diazo group, or an azoxy group. Ch* represents a chiral center.The chiral center is typically carbon (C*). R3 and R5 are each hydrogen,a methyl group, an amino group, a carbonyl group, a carboxyl group, acyano group, an amine group, a nitro group, a nitrile group, or an alkylchain. R3, R4, and R5 are mutually different functional groups.

The liquid crystal material may be a mixture of a plurality of liquidcrystal molecules with different compositions. For example, a mixture ofnematic liquid crystal molecules and smectic liquid crystal moleculesmay be used as the material of the optical waveguide layers 20.

The structure in each of the examples in FIGS. 63 and 65 may be formedby laminating the first mirror 30 and the other components. In thiscase, the structure can be produced easily. When the spacers 73 areformed of a solid material, the first mirror 30 may be formed by, forexample, vapor deposition or sputtering.

In the structural examples in FIGS. 61, 63, and 65, the structure ofeach first mirror 30 has been described on the assumption that theplurality of waveguide elements 10 share the second mirror 40. Ofcourse, the above discussion is applicable to the second mirror 40.Specifically, when the width of at least one of the first and secondmirrors 30 and 40 in the Y direction is larger than the width of theoptical waveguide layers 20, leakage of evanescent light from theoptical waveguide layers 20 can be prevented. A reduction in the amountof light used for optical scanning can thereby be prevented.

Application Examples

FIG. 67 is an illustration showing a structural example of an opticalscanning device 100 including elements such as an optical divider 90, awaveguide array 10A, a phase shifter array 80A, and a light source 130integrated on a circuit substrate (i.e., a chip). The light source 130may be a light-emitting element such as a semiconductor laser. The lightsource 130 in this example emits single-wavelength light with awavelength of X in free space. The optical divider 90 divides the lightfrom the light source 130 and introduces the resulting light beams intoa plurality of waveguides of a plurality of phase shifters. In thestructural example in FIG. 67, an electrode 62 a and a plurality ofelectrodes 62 b are provided on the chip. A control signal is suppliedto the waveguide array 10A from the electrode 62 a. Control signals aresent from the plurality of electrodes 62 b to the plurality of phaseshifters 80 in the phase shifter array 80A. The electrodes 62 a and 62 bmay be connected to an unillustrated control circuit that generates theabove-described control signals. The control circuit may be disposed onthe chip shown in FIG. 67 or on another chip in the optical scanningdevice 100.

By integrating all the components on the chip as shown in FIG. 67,optical scanning over a wide area can be implemented using the smalldevice. For example, all the components shown in FIG. 67 can beintegrated on a chip of about 2 mm×about 1 mm.

FIG. 68 is a schematic diagram showing how two-dimensional scanning isperformed by irradiating a distant object with a light beam such as alaser beam from the optical scanning device 100. The two-dimensionalscanning is performed by moving a beam spot 310 in horizontal andvertical directions. By combining the two-dimensional scanning with awell-known TOF (time of flight) method, a two-dimensional range imagecan be obtained. In the TOF method, a target object is irradiated with alaser beam, and the reflected light is observed. The time of flight ofthe light is computed, and the distance is thereby determined.

FIG. 69 is a block diagram showing a structural example of a LiDARsystem 300 that is an example of a photodetection system capable ofgenerating a range image. The LiDAR system 300 includes the opticalscanning device 100, a photodetector 400, a signal processing circuit600, and a control circuit 500. The photodetector 400 detects lightemitted from the optical scanning device 100 and reflected from thetarget object. For example, the photodetector 400 may be an image sensorsensitive to the wavelength λ of the light emitted from the opticalscanning device 100 or a photodetector including light-receivingelements such as photodiodes. The photodetector 400 outputs an electricsignal corresponding to the amount of the light received. The signalprocessing circuit 600 computes the distance to the target object basedon the electric signal outputted from the photodetector 400 andgenerates distance distribution data. The distance distribution data isdata indicating a two-dimensional distance distribution (i.e., a rangeimage). The control circuit 500 is a processor that controls the opticalscanning device 100, the photodetector 400, and the signal processingcircuit 600. The control circuit 500 controls the timing of irradiationwith the light beam from the optical scanning device 100, the timing ofexposure of the photodetector 400, and the timing of signal reading andinstructs the signal processing circuit 600 to generate a range image.

In the two-dimensional scanning, a frame rate for acquisition of rangeimages can be selected from 60 fps, 50 fps, 30 fps, 25 fps, 24 fps, etc.often used for general video images. In consideration of application tovehicle-mounted systems, the higher the frame rate, the higher thefrequency of range image acquisition, and the higher the accuracy ofobstacle detection. For example, when the frame rate is 60 fps and avehicle is driving at 60 km/h, an image can be acquired every time thevehicle moves about 28 cm. When the frame rate is 120 fps, an image canbe acquired every time the vehicle moves about 14 cm. When the framerate is 180 fps, an image can be acquired every time the vehicle movesabout 9.3 cm.

The time required to acquire one range image depends on a beam scanningspeed. For example, to acquire an image with 100×100 resolvable pointsat 60 fps, each point must be scanned with the beam in 1.67 μs or less.In this case, the control circuit 500 controls the emission of the lightbeam from the optical scanning device 100 and signal accumulation andreading by the photodetector 400 at an operating speed of 600 kHz.

<Examples of Application to Photoreceiver Device>

The optical scanning device of the present disclosure can also be usedas a photoreceiver device having approximately the same structure as theoptical scanning device. The photoreceiver device includes the samewaveguide array 10A as that in the optical scanning device and a firstadjusting element 60 that adjusts a light-receivable direction. Each ofthe first mirrors 30 of the waveguide array 10A allows light incident inthe third direction on a side opposite to a first reflecting surface topass through. Each of the optical waveguide layers 20 of the waveguidearray 10A propagates the light transmitted through a corresponding firstmirror 30 in the second direction. The first adjusting element 60changes at least one of the refractive index and thickness of theoptical waveguide layer 20 of each of the waveguide elements 10, and thelight-receivable direction can thereby be changed. The photoreceiverdevice may further include: the same phase shifters as the plurality ofphase shifters 80 or 80 a and 80 b in the optical scanning device; and asecond adjusting element that changes the phase differences betweenlight beams outputted from the plurality of waveguide elements 10through the plurality of phase shifters 80 or 80 a and 80 b. In thiscase, the light-receivable direction can be changed two dimensionally.

For example, by replacing the light source 130 in the optical scanningdevice 100 shown in FIG. 67 with a receiving circuit, a photoreceiverdevice can be configured. When light with a wavelength λ enters thewaveguide array 10A, the light is transmitted to the optical divider 90through the phase shifter array 80A, combined into one beam, and sent tothe receiving circuit. The intensity of the one combined beam representsthe sensitivity of the photoreceiver device. The sensitivity of thephotoreceiver device can be adjusted by an adjusting element installedin the waveguide array and another adjusting element installed in thephase shifter array 80A. In the photoreceiver device, the direction ofthe wave vector shown in, for example, FIG. 26 (the thick arrow) isreversed. The incident light has a light component in the extendingdirection of the waveguide elements 10 (the X direction) and a lightcomponent in the arrangement direction of the waveguide elements 10 (theY direction). The sensitivity to the light component in the X directioncan be adjusted by the adjusting element installed in the waveguidearray 10A. The sensitivity to the light component in the arrangementdirection of the waveguide elements 10 can be adjusted by the adjustingelement installed in the phase shifter array 80A. θ and α₀ (formulas(16) and (17)) can be determined from the phase difference Δϕ betweenthe light beams when the sensitivity of the photoreceiver device ismaximized and the refractive index n_(w) and thickness d of the opticalwaveguide layers 20. This allows the incident direction of the light tobe identified.

The above-described embodiments and modifications can be appropriatelycombined. For example, the first mirrors 30 shown in FIG. 61 may be usedinstead of the first mirrors 30 in a different embodiment or amodification. The waveguide elements 10 shown in FIG. 65 may be usedinstead of the waveguide elements 10 in a different embodiment or amodification.

The optical scanning device and the photoreceiver device in theembodiments of the present disclosure can be used for applications suchas LiDAR systems installed in vehicles such as automobiles, UAVs, andAGVs.

What is claimed is:
 1. An optical scanning device comprising: an opticalwaveguide layer that propagates light supplied to the optical waveguidelayer; a first mirror having a first reflecting surface that faces theoptical waveguide layer; and a second mirror having a second reflectingsurface that faces the first reflecting surface of the first mirror withthe optical waveguide layer disposed therebetween, wherein the width ofthe first mirror in a first direction and the width of the second mirrorin the first direction are each larger than the width of the opticalwaveguide layer in the first direction, and the first mirror isconfigured to allow a part of the light propagating through the opticalwaveguide layer to pass through and be emitted.
 2. The optical scanningdevice according to claim 1, wherein the optical waveguide layer has avariable thickness and/or a variable refractive index for the light, andthe refractive index of the optical waveguide layer and/or the thicknessof the optical waveguide layer is changed to change an emissiondirection of the part of the light.
 3. The optical scanning deviceaccording to claim 2, further comprising a first adjusting element thatchanges the refractive index of the optical waveguide layer and/or thethickness of the optical waveguide layer to change the emissiondirection of the part of the light.
 4. The optical scanning deviceaccording to claim 3, wherein the optical waveguide layer of thewaveguide contains a first material whose refractive index for the lightis changed when a voltage is applied, and the first adjusting elementincludes a pair of electrodes sandwiching the optical waveguide layerand applying the voltage to the pair of electrodes.
 5. The opticalscanning device according to claim 4, wherein the first materialcontains an electrooptical material.
 6. The optical scanning deviceaccording to claim 4, wherein the first material contains a liquidcrystal material.
 7. The optical scanning device according to claim 1,wherein at least one of the first and second mirrors includes adielectric multilayer film.
 8. The optical scanning device according toclaim 1, further comprising a phase shifter that includes a waveguideconnected to the optical waveguide layer directly or indirectly throughanother waveguide.
 9. An photoreceiver device comprising: an opticalwaveguide layer that propagates light supplied to the optical waveguidelayer; a first mirror having a first reflecting surface that faces theoptical waveguide layer; and a second mirror having a second reflectingsurface that faces the first reflecting surface of the first mirror withthe optical waveguide layer disposed therebetween, wherein the width ofthe first mirror in a first direction and the width of the second mirrorin the first direction are each larger than the width of the opticalwaveguide layer in the first direction, and the first mirror isconfigured to allow a part of light incident on a surface of the firstmirror opposite to the first reflecting surface to be introduced intothe optical waveguide layer.
 10. The photoreceiver device according toclaim 9, wherein the optical waveguide layer has a variable thicknessand/or a variable refractive index for the light, and the refractiveindex of the optical waveguide layer and/or the thickness of the opticalwaveguide layer is changed to change a receivable direction in which thepart of the light is receivable.